Rename Codex to CDX (writer, code symbols, docs)

[gvonnessi]

Tracks the upstream Codex → CDX ("Content-addressed Document eXchange") spec rename (spec PR merged). Makes cdx-pandoc fully consistent with the new name.

Functional / on-the-wire identifiers

• Manifest spec-version key "codex" → "cdx" in emitted JSON.
• MIME type application/vnd.codex+json → application/vnd.cdx+json.
• Extension IDs codex.{academic,semantic,...} → cdx.<same> (e.g. cdx.academic, cdx.semantic).

Writer file rename

• git mv codex.lua → cdx.lua (the Pandoc custom writer). The invocation changes: pandoc ... -t codex.lua is now pandoc ... -t cdx.lua.
• Updated every reference: Makefile (WRITER := cdx.lua, .manifest.cdx), README/CONTRIBUTING usage examples, and scripts/pandoc-to-cdx.sh (CODEX_WRITER → CDX_WRITER plus the writer path string).
• This matches the existing cdx-reader.lua naming convention.

Code symbols

• Internal Lua identifiers renamed: CODEX_VERSION → CDX_VERSION, codex_field/codex_name → cdx_*, and codex-referencing comments throughout lib/.

Docs / prose

• Brand Codex → CDX in README, CHANGELOG, CONTRIBUTING, SECURITY, and the LICENSE copyright holder lines ("CDX Document Format Contributors").

Tests

• Golden baselines under tests/expected/ updated to the new key / MIME / extension IDs so golden and round-trip tests pass.

Preserved

• The upstream spec repo path token codex-file-format-spec is intentionally left unchanged (it is still named that). A re-grep confirms it is the only remaining codex token in the tree.

Verification (local, macOS)

Installed pandoc 3.10, lua 5.4.8, luacheck 1.2.0, jq 1.7.1 and ran:

• make lint — 0 warnings / 0 errors
• make test — all unit suites + JSON output pass
• make test-golden — all golden tests pass
• make test-reader — all round-trip tests OK
• make validate — structure OK
• make test-all — passes (exit 0)
• make test-cdx — full .cdx packaging pipeline OK; generated manifest.json contains "cdx": "0.1"