Protocol plugin architecture: MySQL/PostgreSQL tracing (deferred, see ROADMAP.md P4-4)

[epli2]

Tracking issue for deferred protocol-plugin work, split out of #2 (closed).

Background

#2 implemented MySQL wire-protocol tracing via the LD_PRELOAD backend: crates/phantom-core/src/mysql.rs, crates/phantom-storage/src/fjall_mysql.rs, plus TUI display support. It's a substantial, working implementation, but was closed rather than merged — see the closing comment on #2 for the full rationale. Summary:

1. Concept fit: the current ROADMAP.md (§2) redefines Phantom as an HTTP-focused "local-first API development toolbox" (observe/perturb/record-replay/spec-gen). Landing a second protocol before the HTTP core (persistent CA, compression, TUI usability, JSONL schema versioning) is finished would double maintenance surface prematurely.
2. Design mismatch: Add MySQL trace functionality #2 added HttpTrace-parallel types rather than a shared event model, and is LD_PRELOAD-only (Linux, no TLS), asymmetric with the primary proxy backend.
3. Staleness: based on a main several months old; would need a full rebase against current src/main.rs / phantom-tui.

What to do when this is picked back up

Per ROADMAP.md P4-4, this should not be re-merged as-is. Prerequisite design work:

• Generalize phantom_core::trace::HttpTrace into something like enum TraceEvent { Http(HttpTrace), Db(DbTrace), … } (or a trait-based equivalent) so storage, TUI, and JSONL output can handle event kinds without hardcoding HTTP.
• Write an RFC (docs/rfcs/NNN-protocol-plugins.md) covering the event model change before implementation.
• Decide capture-backend story for DB protocols on the (cross-platform) proxy backend, not just LD_PRELOAD.
• gRPC (HTTP/2-based) is called out in the roadmap as a lower-effort first protocol-plugin candidate, since it can extend the existing proxy rather than requiring a new backend.

Reference

• Original implementation: Add MySQL trace functionality #2 (branch claude/add-mysql-trace-m434f)
• Roadmap items: ROADMAP.md §1.2 (PR evaluation), §9 ADR-1, task P4-4

Gate: per ROADMAP.md §9, Phase 4 items (including this one) should only be picked up after Phase 3 completes and there's an external signal of demand.