Add symbol_meta reader for per-symbol geometry sidecars

Implements #3: DataServer::symbol_meta(symbol) -> Option<InstrumentGeometry>
loads a <SYMBOL>.meta.json sidecar from the owned data directory and returns
neutral, broker-agnostic price geometry (digits, pip_size, tick_size,
lot_size, base, quote). A consumer can now obtain a symbol's bars and its
price-interpretation geometry from one server.

Design (forks logged on #3):
- Neutral output is the struct's shape/semantics, not currency-code
  normalization: base/quote pass through verbatim. The live sidecars carry
  wildly heterogeneous base codes (ISO, index codes, descriptive names, stock
  tickers) with no definable neutral target; the downstream consumer
  cross-checks them against its own vetted table.
- Format quirks are absorbed in a pure parser (meta::from_sidecar_bytes):
  scientific notation parses natively; a null / non-finite / missing core
  field or an unsupported schemaVersion yields None, never a partial struct.
- A missing sidecar is a normal None (silent); a present-but-unusable one
  warns to stderr, matching the existing loader's degrade-and-warn style.
- serde_json (Value extraction, no derive) is the new direct dependency,
  chosen over a hand-rolled parser for robust number/escape handling; the
  README dependency statement is updated to match.
- No caching and no Arc<Self>: geometry files are tiny and read rarely, and
  the read needs no prefetch. Lookup is not gated on bar-index membership.

Verified: cargo build and clippy clean (0 warnings); full suite green
(7 parser unit tests + 5 symbol_meta integration tests pin each absorbed
quirk and the index-independent lookup). Cargo.lock stays untracked per the
library convention.

closes #3

Cargo.toml

@@ -7,6 +7,7 @@ description = "High-performance, thread-safe loader and cache for binary market
 [dependencies]
 chrono = "0.4"
 regex = "1.10"
+serde_json = "1"
 zip = "2"
 
 [dev-dependencies]