Expose semantic MCP search and bounded repository find

[Widthdom]

Adds CLI semantic filtering to MCP search and regex find_in_file, and introduces find for bounded repository-wide scans. Clients can resume scan-, row-, or byte-limited pages without losing matches; the existing required-path contract remains in place for find_in_file.

• Share validation, lexical classification, scanners, cursor codecs, and scan metadata with the existing query engine. Reject changed filters and stale generations while retaining unknown-origin and partial-result authority signals.
• Keep discovery, validation, STDIO, HTTP, and batch_query synchronized.
• Update the English/Japanese user and testing documentation and add changelog.d/unreleased/5349.added.md.

Validation:

• Release solution build: zero warnings/errors.
• Release MCP, HTTP, catalog, shared search/find, and C# origin-continuation regressions: .NET 8 — 1,481 passed; .NET 9 — 1,480 passed, one existing skip.
• make lint, all seven changelog fragment checks, and git diff --check passed.
• Real STDIO server smoke: discovery, semantic search, scan-cap continuation with a changed line budget.
• Two Codex adversarial review rounds of origin/main..HEAD; all five findings addressed and regression-tested, including the distinction between actual candidate caps and page fullness.

Follow-up: #5360.

Fixes #5349