edge: add Top-of-Book parser framework

[armcconnell]

Summary

• Introduces the edge package with a pluggable Parser interface, a parser registry, and the first implementation: a decoder for the DoubleZero Top-of-Book feed v0.1.0 wire format.
• Parser is stateful: it learns instrument definitions from refdata messages and uses them to enrich marketdata (quote/trade) messages with symbol, price/qty exponents, and derived floats. Marketdata arriving before its definition is held in a bounded per-instrument buffer (most-recent-wins, capped at 1000 instruments) and flushed the moment the definition lands, so a cold-start reader still produces meaningful output at the first refdata cycle.
• Wire decoder is a hand-written positional reader over the fixed little-endian format using `encoding/binary` and a small sticky-error reader helper. No third-party deps, no code generation.
• Emits generic `Record` values; intentionally decoupled from output sinks, which land in the next PR.

Part 1 of 4 — edge feed parser stack

1. this PR — edge: add Top-of-Book parser framework ← start here
2. edge: add JSON/CSV/Unix-socket output sinks
3. edge: add feed runner, manager, HTTP API, and CLI
4. edge: add integration tests, synthetic publisher, and devnet guide

Each PR compiles and tests standalone. Feature is off-by-default until PR 3 lands; early PRs are dormant code.

Testing Verification

• Unit tests cover wire decode for each message type, bounded-buffer overwrite semantics, cap-reached drops, and refdata-triggered flush.
• Also exercised against real testnet Top-of-Book traffic on a QA node — processed 1M+ records end-to-end without buffer growth or parse errors.

Size note

~1000 lines of production code, above doublezero's ~500-line guideline. The stack already splits the feature into 4 PRs; the parser itself doesn't subdivide further without creating non-functional intermediate states. Bulk of the diff is the wire decoder (~340 LOC, straight-line positional reads) and the parser state machine (~600 LOC), both of which benefit from being reviewed together.