feat(mediainfo): add MediaInfoContext for reusable library loading

[bakgio]

Why

Every v0.1.0 parse call paid a full dlopen → 10-symbol resolve → MediaInfo_New → version probe → configure → parse → MediaInfo_Delete → dlclose cycle. For single-file use this is invisible; for batch workloads (asset scanners, directory walks, import pipelines) the load overhead dominates and is pure waste.

This PR adds a reusable MediaInfoContext that loads the MediaInfo shared library once and reuses it across many parse calls. Existing free functions are transparently routed through a lazily initialized process-wide default context so current users get the performance win without a single line of code change. The feature is purely additive — no public API signature changed and every existing test passes unmodified.

What

[core] src/mediainfo.rs

• New public pub struct MediaInfoContext (Clone + Send + Sync) with the full 16-method parse surface mirrored from MediaInfo (parse, parse_media_info, parse_path, parse_from_reader, parse_input, their _with_options variants, plus the two parse_to_string / parse_reader_to_string raw-text helpers).
• Three constructors: new() (default search order), with_library_file(path), with_library_search_dir(dir).
• Accessors: library_version() -> LibVersion, library_version_string() -> &str, library_file() -> Option<&Path>, can_parse() -> bool.
• Private internal refactor:
  • New struct LoadedLibrary<'a> carrying a borrowed &'a Arc<MediaInfoLib> + cached version info.
  • load_library split into load_library_full(library_file, library_search_dir) -> Result<(Arc<MediaInfoLib>, String, LibVersion)>.
  • parse_to_string_internal_unlocked, parse_to_string_from_url_unlocked, parse_reader_to_string_internal_unlocked, and parse_url_via_http now take &LoadedLibrary<'_> as their first parameter instead of loading the library themselves.
• Process-wide default context: static DEFAULT_CONTEXT: OnceLock<RwLock<Option<Arc<MediaInfoContext>>>>, lazy-initialized on first use, resettable.
• New pub fn MediaInfo::reset_default_context() escape hatch to drop and rebuild the cached context.
• Each locked wrapper (parse_to_string_internal, parse_to_string_from_url, parse_reader_to_string_internal) now checks whether options.library_file/options.library_search_dir is set:
  • If yes → fresh per-call load_library_full (v0.1.0 path preserved verbatim).
  • If no → borrow the shared library from the default context.
• ParseOptions::mediainfo_options rustdoc gained a "Thread safety" subsection explaining how custom options interact with the process-wide parse lock.

[errors] src/error.rs

• New variant MediaInfoError::LibraryMismatch { context: PathBuf, requested: PathBuf } plus a library_mismatch(context, requested) constructor helper.
• Returned when a context parse call is given a library_file / library_search_dir override that would require a different library than the one the context already loaded. Matching library_file is allowed (tautology); library_search_dir always triggers a mismatch because a re-resolve is conceptually a different load.

[api] src/lib.rs

• Re-export MediaInfoContext from the crate root alongside the existing types.
• Crate module docs updated with a MediaInfoContext feature bullet and a pointer to the new batch_parse example.

[tests] 20 new tests, total count 203 → 223

• tests/context_tests.rs (new, 14 tests): basic reuse, equivalence with the free function path, version cache, shared-across-threads stress (16 workers × 5 parses), library mismatch (file + search-dir), matching-file tautology, reader input, raw JSON output, custom options isolation between calls, default context reuse, reset_default_context recovery, free function with explicit library override, and pre-built MediaInfoInput dispatch.
• tests/end_to_end_tests.rs: adds test_context_url_parse (URL via shared context using the existing tiny_http mock) and test_thread_safety_context (100-thread stress with a shared Arc<MediaInfoContext>). Original test_thread_safety is untouched.
• tests/error_unit_tests.rs: 2 new tests covering the LibraryMismatch Display format and variant matching.
• 2 new doctests on MediaInfoContext and MediaInfoError::library_mismatch.

[bench] benches/parse_overhead.rs (new)

• Criterion benchmark with three variants:
  1. free_fn/parse_media_info_path — exercises the default-context fast path.
  2. context/parse_media_info_path — explicit MediaInfoContext reuse.
  3. free_fn/parse_media_info_path_uncached — pins library_file in ParseOptions to bypass the default context and reproduce the v0.1.0 fresh-load path, giving a proper A/B baseline without needing a git worktree.
• criterion = "0.5" added to [dev-dependencies]; new [[bench]] entry in Cargo.toml.

[examples] examples/batch_parse.rs (new)

• Short, argv-driven example mirroring the style of parse_path.rs. Shows MediaInfoContext::new()? + loop, printing the loaded library version and a per-file track count.

[docs] README.md, CHANGELOG.md

• README.md: new "Reusable context" bullet in the features list, examples pointer updated to mention batch parsing, install snippet bumped "0.1.0" → "0.2.0".
• CHANGELOG.md: new 0.2.0 entry with Added / Changed sections.

[ci] .github/workflows/ci.yml

• New semver job that installs cargo-semver-checks --locked and runs cargo semver-checks -p rsmediainfo. Sets RS_MEDIAINFO_SKIP_DOWNLOAD=1 so it does not fetch MediaInfo.dll during the check.
• semver added to the release job's needs: list so a semver regression blocks a release.

[version] Cargo.toml, README.md, .github/ISSUE_TEMPLATE/bug_report.yml

• Crate version bumped 0.1.0 → 0.2.0 in Cargo.toml.
• Version placeholder in the bug-report issue template bumped to match.