docs: comment-toggle framing (Arc 2)

Q#CT1-6: pure Lua on the Arc 2 substrate (effective-edit mutators,
command boundaries, after-edit delivery). M-; = comment-line-style
toggle (named deviation from comment-dwim); pmacs.comment.strings
public table; min-indent insertion, skip-blank, mixed-span comments;
single buf:replace per toggle = one undo step / one CRDT op / one
effective-edit verification.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

docs/comment-toggle-framing.md

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+# Comment/uncomment — framing (Arc 2, editing table stakes)
+
+pmacs has no way to comment code out. This adds the table-stakes
+toggle: `M-;` comments or uncomments the current line or the selected
+lines, language-aware, as one undoable edit.
+
+Roadmap: `docs/roadmap-2026-07.md` Arc 2 ("comment/uncomment").
+
+## Ground truth (as of `2dde4b8`)
+
+- **No comment-syntax knowledge exists anywhere** — not in the grammar
+  registry, not in LSP config, not in Lua. A language → prefix table is
+  new surface.
+- **Language detection**: `active_buffer_language()` in
+  `builtin/runtime/lsp.lua` chains grammar detection
+  (`pmacs.parse.language_for_path`) with the user-extensible
+  `pmacs.lsp.filetypes` map — but it is a **local**. Known languages
+  today: rust/lua/c/cpp + js/ts via grammars; python, c, go,
+  tsx/jsx, lua, bash, toml, zig via filetypes.
+- **Bindings**: `M-;` (Emacs `comment-dwim`'s home) is free. `C-/` is
+  taken by undo (terminal `Ctrl+/` ambiguity — three undo bindings
+  exist), so the VSCode-style toggle chord is unavailable.
+- **Undo granularity**: every applied edit pushes one `UndoEntry`
+  (`src/buffer.rs:130`) — there is no grouping/transaction. N per-line
+  edits would need N undos.
+- From Arc 2 (merged): the buffer mutators return the **effective
+  post-intercept edit** `(start, end, inserted_len)`; keybound commands
+  rotate the command boundary and get `buffer.after-edit` from
+  dispatch; `M-x` gets both via `invoke_interactive` +
+  `with_after_edit_check`. Comment-toggle inherits all of it for free.
+
+## Decisions
+
+### Q#CT1 — Pure Lua, zero Rust changes
+
+`builtin/runtime/comment.lua`. Everything needed exists: `buf:slice`,
+the effective-edit-returning `buf:replace`, `ed.region/cursor/
+goto_byte/clear_selection`, and the language chain. The one lsp.lua
+touch: **export** the existing local as
+`pmacs.lsp.active_buffer_language()` (one line) rather than replicating
+its grammar+filetypes chain and drifting.
+
+### Q#CT2 — Command + binding
+
+One command, `edit.toggle-comment`, bound **`M-;`**:
+
+- **Region active** → toggle the whole lines the region touches, then
+  clear the selection (CUA convention after a region op) and leave the
+  cursor at the span start.
+- **No region** → toggle the current line, then **move to the next
+  line** — Emacs's `comment-line` behavior, which makes repeated `M-;`
+  walk down a block toggling as it goes.
+
+Named deviation from Emacs: `M-;` in Emacs is `comment-dwim`, whose
+no-region case *appends* an empty comment at end of line. That mode is
+rarely what modern muscle memory wants from the toggle key; pmacs's
+`M-;` behaves like Emacs `C-x C-;` (`comment-line`). DWIM's
+append-comment can come later under its own name.
+
+### Q#CT3 — The comment-string table
+
+`pmacs.comment.strings` — a public, user-extensible map (the
+`pmacs.lsp.filetypes` pattern), language → line-comment prefix:
+
+`//`: rust, c, cpp, go, zig, javascript, typescript,
+javascriptreact, typescriptreact · `--`: lua · `#`: python, bash,
+toml, yaml, sh.
+
+Unknown language (or no language): status *"no comment syntax known
+for `<lang>`"*, no edit. Users add entries from init.lua:
+`pmacs.comment.strings.mylang = ";;"`. **Block comments are deferred**
+— line comments cover the table-stakes use, and block toggling has
+real edge cases (nesting, mid-line spans) that don't belong in v1.
+
+### Q#CT4 — Toggle semantics
+
+Over the span's lines:
+
+- **Uncomment** when every non-blank line starts (after its
+  indentation) with the prefix; removal strips the prefix plus one
+  following space if present.
+- **Comment** otherwise: insert `prefix + " "` at the **minimum
+  indentation column** of the span's non-blank lines (Emacs
+  `comment-region` style — the comments line up instead of hugging
+  each line's own indent). **Blank lines are skipped** in both
+  directions and don't influence the min-indent computation.
+- A span that is entirely blank is a no-op with a status.
+
+Mixed spans (some commented, some not) therefore **comment** — the
+double-prefix on already-commented lines round-trips back out, which
+is Emacs's behavior and preserves inner commented-out code.
+
+### Q#CT5 — One edit, one undo step, one CRDT op
+
+The whole toggle is a **single `buf:replace(span_start, span_end,
+new_text)`**: Lua builds the rewritten span, one edit applies it.
+Consequences, all deliberate:
+
+- **One `C-/` undoes the whole toggle** (there is no undo grouping to
+  lean on; N per-line edits would need N undos).
+- One CRDT op for replica frontends.
+- One effective-edit verification: the kill-ring discipline — if the
+  returned `(start, end, inserted_len)` deviates from the request (a
+  buffer intercept rewrote it), report *"comment toggle altered by
+  buffer intercept"* and skip the cursor fix-up; the interceptor's
+  result stands. `pcall`'d, so a rejecting intercept reports rather
+  than throws.
+
+### Q#CT6 — Chain/hook plumbing: nothing to build
+
+Keybound `M-;` rotates the command boundary (breaking kill chains —
+correct) and fires `buffer.after-edit` from dispatch's revision check;
+`M-x edit.toggle-comment` gets the same via `invoke_interactive` and
+the accept-path hook wrapper. Both are the Arc 2 substrate working as
+designed — the acceptance suite asserts the hook fires once anyway.
+
+## Bets
+
+1. **Single-replace is the right granularity** — no complaint about
+   whole-span replaces (vs. per-line edits) from CRDT replicas or LSP
+   didChange (full-text sync makes this moot today).
+2. **Min-indent + skip-blank matches expectation** — no "why is my
+   comment at column 0" or "why did my blank line get a `//`".
+
+## Deferred (named)
+
+- Block comments (`/* */`) and mid-line spans.
+- `comment-dwim`'s append-comment-at-EOL mode.
+- Doc-comment continuation on newline (belongs to auto-indent).
+- Per-language *padding* config (always one space in v1).
+
+## Acceptance (`tests/comment_toggle_acceptance.rs`, dispatch-driven)
+
+- Rust buffer: `M-;` comments the line (`// ` at indent), cursor moves
+  to the next line; `M-;` on a commented line uncomments (round-trip,
+  including the space).
+- Region across mixed-indent lines → prefixes at min indent, aligned;
+  blank line inside the span untouched; selection cleared.
+- All-commented span → uncomments; mixed span → comments (inner
+  prefix preserved, round-trips).
+- Lua buffer gets `--`, Python `#`; unknown/no language → status, no
+  edit.
+- **One undo step**: multi-line toggle then a single `buffer.undo`
+  restores the original text exactly.
+- Intercept discipline: rejecting intercept → status, no throw;
+  transforming intercept → reported, no cursor fix-up.
+- `after-edit` probe fires exactly once per toggle (keybound and
+  `M-x`).
+- Kill-chain break: `C-k`, `M-;`, `C-k` → two ring entries.