Chameleon

Cross-file existence breaks can now BLOCK at the Stop backstop (roadmap #10),
opt-in per repo. Removing a named export that other files still import is exactly
the defect a human reviewer catches, and the engine already detected it — but
only as an advisory the model could ignore. When enabled, a turn-introduced,
live-re-verified export removal now refuses the turn.

Added

• Cross-file existence deny (opt-in; enforcement.crossfile_existence_block,
  default false). When enabled and enforcement.mode is enforce, a Stop where
  the turn removed a named TypeScript/Python export an indexed importer still
  references BLOCKS the turn, naming the removed export and its broken call sites;
  mode: shadow logs a would_block row instead. Stop-only, never inline
  (PreToolUse/PostToolUse are untouched). Overridable with a
  chameleon-ignore removed-export-breaks-importers comment in the edited source
  and bounded by stop_block_cap. Off by default so an existing enforce repo
  gains no new block class on upgrade; the default-on cross-file advisory still
  surfaces the same breaks, so a team can watch them before opting the deny on.

  The deny is engineered false-positive-free: every block is (1) re-verified live
  against the working tree at Stop, so a break the model fixed later in the same
  turn never blocks; (2) scoped to a removal introduced THIS turn, by confirming
  the export existed at git HEAD, so a pre-existing broken import is never blamed
  on the turn that merely edited the file; (3) confirmed per importer to still
  source the name from the target, so a same-turn repoint to another module or a
  bare package does not block; and (4) suppressed when the target still provides
  the name via a re-export or a CommonJS conversion. Ruby constants, barrels, and
  deleted modules stay advisory-only — under-blocking is the safe direction for a
  deny. See _confirmed_crossfile_break_sites in hook_helper.py.

Fixed

• Comment-defeats-repoint over-block in cross-file import-source resolution: a
  commented-out stale import (// import { foo } from './old' left behind after
  repointing foo elsewhere) was parsed as a live binding, re-introducing the old
  target and firing a phantom break. _imported_source_keys now blanks comments
  (keeping string specifiers intact) before scanning, via a new comment-only mode
  on the string/comment tokenizer. The advisory tolerated this as noise; the new
  deny could not.