team: deterministic prefilter for findings outside the diff

[pierrick-fonquerne]

Scope

Synthesised findings sometimes point at a file line that belongs to no hunk of that file's patch. Such findings describe code this pull request does not touch, yet each one costs three lens calls before being contested.

This change adds a deterministic prefilter between synthesis and the lens vote:

• github.rs: unified diff hunk range parsing (parse_hunks, parse_hunk_header) and DiffContext::line_in_patch, which reports whether a finding's line falls inside a hunk of its file patch
• team.rs: prefilter_verdict contests a finding whose line is provably outside the diff, with a deterministic reason rendered in the contested section; verify_findings skips the lens calls for prefiltered findings

Findings with line 0, findings on files without a per-file patch, and findings anchored inside a hunk range keep the current behaviour.

Acceptance criteria

• A finding whose line is outside all hunks of its file patch is contested without any lens call
• Findings with line 0, without a per-file patch, or inside a hunk range keep the current behaviour
• The deterministic reason is rendered in the contested section of the comment
• Hunk parsing covered by unit tests (header with and without count, malformed headers, multi-hunk ranges)

26 tests pass, clippy clean (all + pedantic).

Closes #8