Fix case-only filename collisions silently overwriting Obsidian/Canvas/Wiki notes

[TPAteeq]

Summary

Closes #1453.

Three exporters derive on-disk filenames from node/community labels and
dedup them, but keyed the dedup on the exact-case name. On
case-insensitive filesystems — macOS/APFS and Windows/NTFS, the two most
common desktop targets — two labels that differ only by case (e.g. a class
References and a markdown heading references) are distinct keys but
resolve to the same path on disk, so the second write_text() silently
overwrites the first: no numeric suffix, no warning, no signal in the
return value. Same data-loss class as #497.

The reporter confirmed it empirically: a 3,288-node graph wrote 3,366 files
against an expected 3,489 — 123 notes silently lost.

Fix

• to_obsidian / to_canvas (graphify/export.py): the node-note
  dedup now keys a used set on base.lower() while still emitting the
  original-case filename, so a real collision gets a numeric suffix. The
  dedup was duplicated verbatim across both functions (the issue flagged the
  drift risk), so it's extracted into one shared _dedup_node_filenames()
  helper.
• to_wiki (graphify/wiki.py): _unique_slug had the same
  case-sensitive slug in used_slugs membership (the wiki.py: _safe_filename(label) collisions cause silent article overwrites #497 dedup didn't fold
  case); it now folds case the same way.
• Review-driven hardening (same PR):
  • to_obsidian's _COMMUNITY_*.md overview notes had no dedup at all,
    so two community labels differing only by case — or even identical after
    sanitizing — clobbered each other. One case-folded-deduped filename per
    community is now computed up front and used for both the write and the
    intra-vault [[_COMMUNITY_...]] cross-references so they stay consistent.
  • The generated suffix is itself re-checked, so a synthesized base_1
    can't silently overwrite a node whose literal label is already base_1
    (this collides on case-sensitive filesystems too).

Testing

• Regression tests in tests/test_export.py and tests/test_wiki.py,
  robust on both filesystem types: each asserts every note lands on disk
  (catches the overwrite on case-insensitive FS) and that no two stems
  collide under .lower() (catches the missing suffix on case-sensitive FS).
  Coverage: obsidian/canvas case collision, literal-suffix collision,
  community-note collision, obsidian↔canvas filename agreement (the real CLI
  path calls them separately), wiki community↔community and god-node↔community
  collisions. Each new test verified red on the pre-fix code, green after.
• Full test_export.py / test_wiki.py plus the adjacent
  obsidian/canvas/CLI/pipeline suites pass (99 tests).
• Verified end-to-end in Obsidian: a graph with References/references
  and Parser/parser/PARSER produces five distinct notes
  (References, references_1, Parser, parser_1, PARSER_2) and a
  canvas whose cards each resolve to a real file.

Out of scope (follow-ups)

• report.py's GRAPH_REPORT.md emits its own [[_COMMUNITY_...]] hub
  links via a separate sanitizer; for a suffixed (case-colliding) community
  its link can dangle. Pre-existing cross-tool coupling — the vault's own
  notes and internal links are now correct and no longer lose data.
• transcribe.py writes {stem}.txt transcripts with no dedup, so files
  from different dirs with case-only-distinct stems can overwrite. Separate,
  pre-existing, not label-keyed — maybe worth its own issue.

[safishamsi]

Landed on v8 (commit 5d63aad), authored to you. Verified with a rigorous edge-case battery beyond the included tests: case-only collision (References + references_1), the base_1 literal re-check (a synthesized base_1 becomes base_1_1 rather than clobbering a node literally named base_1), community-label case folding (_COMMUNITY_API + _COMMUNITY_api_1), and determinism across runs. Full suite 2404 passed, ruff + skillgen clean. Nicely reasoned fix — the shared helper and the suffix re-check are exactly right. Closing as landed.