feat(scanner): reject malformed UTF-8 plugin manifests

[seonghobae]

Current repair boundary — 2026-09-12

• base: feat/claude-plugin-nonstandard-json-1099@b02bfdecec575edf593a72388df2cd749afdf204
• exact head: fd8673ea21543eb40f05ec0017d20447432d866e
• exact tree: 3b000417cf1883e8930b723194ff0cc83f343568
• ancestry: ordinary two-parent merge of previous feat(scanner): reject malformed UTF-8 plugin manifests #1154 head ee43e7d… and current feat(scanner): reject non-standard plugin JSON constants #1153 head b02bfde…; no force update
• exact-tree validation: PYTHONPATH=. python -m pytest -q tests/test_claude_plugin*.py → 182 passed; py_compile and git diff --check passed
• lifecycle: Draft / malformed-UTF8 delta and all parent repairs preserved / hosted exact-head Checks and independent review absent
• stacked-workflow admission remains owned by fix(ci): admit stacked PR checks without bypassing docs contracts #1096; do not create a source-neutral wake-up commit

Successor of #1153 / issue #1099. Does not Close those. Bidi/control stays the concealment class. Duplicate members stay #1129. Nonstandard JSON constants stay #1153.

Unique delta

Fail closed when marketplace/plugin/MCP JSON bytes are not valid UTF-8 (claude-plugin-malformed-utf8):

• truncated multibyte sequence in plugin.json fails admission
• invalid continuation and lone-surrogate UTF-8 fail admission
• valid Korean/Japanese description is not this finding
• bidi concealment stays claude-plugin-concealed-identity
• feat(scanner): reject non-standard plugin JSON constants #1153 Infinity stays claude-plugin-nonstandard-json-constant
• invalid JSON that is valid UTF-8 stays the JSON-decode class
• snippets are short labels and omit secrets and raw invalid bytes

Test plan

• RED then GREEN (tests/test_claude_plugin_malformed_utf8.py)
• Detector statement coverage 1614/1614 with plugin suites on Python 3.13
• Exact-head Checks on this head
• Keep Draft until current-head gates are GREEN

Relates to #1099. Relates to #1153.

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