perf: stream HAR entries incrementally

[Arkptz]

Summary

Replace the monolithic har::from_slice parser with a streaming HAR entries reader that iterates entries one at a time via a custom JSON scanner. HAR files from long browser DevTools sessions can exceed 100 MiB; this change keeps memory usage constant regardless of input size, matching the streaming behavior already in place for mitmproxy flow files.

• Add stream_har_file() that opens a BufReader, scans forward to the "entries": [ array, and yields each entry object individually via bracket-balanced extraction + serde_json::from_slice
• Replace the temporary read_har_file shim in stream_input with the new streaming iterator
• Keep read_har_file as a thin convenience wrapper (stream_har_file().collect())
• Own minimal StreamingHarEntry structs with #[serde(default)] for resilience against incomplete HARs

Type of Change

• Bug fix (non-breaking)
• New feature (non-breaking)
• Breaking change (fix or feature that would cause existing behavior to change)
• Documentation update
• Refactoring / internal cleanup
• CI / build / tooling

Checklist

• cargo fmt --all clean
• cargo clippy --all-targets --all-features -- -D warnings clean
• cargo test passes locally
• Added or updated tests for the change
• Updated README / CLI help text if user-facing behavior changed
• Added entry to CHANGELOG.md under [Unreleased] (if user-facing)
• Used conventional commit style in commit messages (feat:, fix:, docs:, etc.)

Testing

• All 8 existing HAR tests continue to pass with identical output
• stream_har_does_not_materialize_all: constructs a 20-entry HAR, verifies the iterator yields the first entry before consuming all entries
• stream_matches_read_for_fixtures: runs all three HAR fixtures through both stream_har_file().collect() and read_har_file(), asserts field-by-field equality
• stream_malformed_entry_returns_error: verifies a bad entry mid-stream surfaces as Err without aborting subsequent valid entries
• stream_empty_entries_array: verifies "entries": [] yields zero items without error
• cargo test --test security passes (5/5)
• cargo publish --dry-run succeeds

Related Issues

None