…t and exp_date

- Contract.root -> .symbol, Contract.exp_date -> .expiration on the
  fpss::protocol public type.
- Contract::stock / index / rate / option / option_raw constructors
  rename their first positional parameter to symbol; option() renames
  exp_date to expiration.
- FlatFileRow.root -> .symbol; flat-file CSV / JSONL contract-prefix
  headers go from "root,expiration,..." to "symbol,expiration,...".
- IndexEntry.root -> .symbol, IndexEntry.exp -> .expiration (crate-
  private, but mirrored for consistency).
- tick_schema.toml updates the OptionContract column from root to
  symbol so every generator-emitted SDK surface follows.
- Internal byte-level decoder locals keep the wire-spec names (root /
  exp_date) so the parser still diffs cleanly against the upstream
  binary protocol; the public field assignments use the v3 names.

Closes #467

Update the per-language emitters under crates/thetadatadx/build_support
so the regenerated SDK output uses the v3 vendor field names:

- fpss_events::python emits Contract.symbol / Contract.expiration on
  the pyo3 #[pyclass].
- fpss_events::typescript emits Contract.symbol / Contract.expiration
  on the napi-rs #[napi(object)] struct.
- fpss_events::go_structs emits Contract.Symbol / Contract.Expiration
  on the Go SDK struct.
- fpss_events::ffi_rust + ffi_c emit TdxContract.symbol /
  TdxContract.has_expiration / TdxContract.expiration on the C ABI
  mirror; the converter stages the CString as
  contract_symbol_cstring and reads contract.symbol /
  contract.expiration off the core type.
- ticks::go updates the OptionContract source-expression dispatch to
  emit symbol-keyed Go converters.
- The Go cgo template (next_event_body.go.tmpl) and the C++
  option_contracts_convert template assign .symbol on the Go /
  C++ public types instead of .root.

Closes #467

Carries the v3 surface rename through every committed generator output
plus a handful of hand-written companion files:

- ffi/src: TdxContract.symbol / .has_expiration / .expiration in
  fpss_event_structs.rs; TdxOptionContract.symbol in types.rs; the
  fpss_event_converter passes contract.symbol / contract.expiration
  through to the C ABI structs; streaming.rs tracing fields use
  symbol = %contract.symbol.
- sdks/python/src: regenerated Contract pyclass + OptionContract
  pyclass + tick_arrow column buffers use symbol / expiration.
- sdks/typescript/src + index.d.ts: regenerated Contract napi struct +
  OptionContract napi struct + dispatcher arms use symbol / expiration;
  the TypeScript declaration mirrors the renamed fields.
- sdks/go: regenerated fpss_event_structs.go (Contract.Symbol /
  Contract.Expiration), fpss_event_structs.h.inc, fpss_methods.go,
  tick_converters.go, tick_structs.go (OptionContract.Symbol). The
  hand-written tick_ffi_mirrors.go cOptionContract mirror is updated
  to match the new C ABI field name.
- sdks/cpp: thetadx.h TdxOptionContract.symbol; thetadx.hpp
  OptionContract.symbol with updated layout-asserts comment;
  fpss_event_structs.h.inc TdxContract.symbol /
  TdxContract.has_expiration / .expiration; historical.cpp.inc
  reads c.symbol off the C ABI struct.
- tools/cli, tools/mcp, tools/server: option-contract rendering uses
  the new symbol field; REST + MCP + WS JSON outputs emit
  "symbol" / "expiration" keys on every contract payload; the WS
  format snapshot regression test pins the symbol key.
- crates/thetadatadx/src/frames_generated.rs: regenerated
  OptionContract Arrow / Polars column emitters use col_symbol and
  the "symbol" Field name.

Closes #467

Patch bump on the v8 line (v8 is patch-only on every change). Updates
the workspace + every sub-workspace + every package.json + the cpp
CMakeLists.txt project version, refreshes Cargo.lock everywhere, and
adds the 8.0.25 entry to CHANGELOG.md and docs-site/docs/changelog.md
naming the renamed fields and citing the vendor v3 migration guide.

Doc-site streaming / quickstart pages updated to use contract.symbol /
contract.expiration in the example callbacks; CHANGELOG history is
preserved verbatim (v8.0.0 release notes still describe the
historical contract.root surface).

Closes #467