Schema version 5: platform-independent hash that covers custom types

[facontidavide]

Resolves the remaining item of #73 and the "implementation-defined hash" caveat introduced in #78.

What changes

The schema hash becomes FNV-1a 64 of the schema text with its own ### hash: line removed. Consequences:

• identical on every platform (no more std::hash), so any decoder can recompute and verify it;
• covers everything in the schema, including custom type bodies, so two schemas that differ only inside a struct no longer collide in a sink's hash-to-channel map;
• SCHEMA_VERSION bumps to 5.

Compatibility

• New readers (this header-only parser, the Python decoder) read version 4 files as before, through the declared hash, and can verify version 5 files (check_hash / verify_hash=True).
• Parsers older than this release recompute the version-4 hash themselves and would decode version 5 files wrongly; the version bump makes them fail explicitly with "Wrong SCHEMA_VERSION". PlotJuggler needs its data_tamer dependency bumped for the 2.0 release.

Code

• SchemaTextHash() / ComputeSchemaHash() replace AddFieldToHash (library and parser).
• Hash recomputed at every registration and custom type addition (setup time only).
• Golden test now compares schema.txt byte for byte including the hash and checks Schema::hash == SchemaTextHash(text); fixture regenerated. The Python test verifies the hash independently, so both implementations of FNV-1a agree on the real fixture.
• Spec section 5 rewritten as normative, version history added, changelog entry.

Targets V2 together with #75.

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