fix(validator): close validation gaps and realign schema.json

[jedymatt]

Related issue: #42 — this is the first incremental pass toward the validator rewrite tracked there (does not close it).

Summary

The hand-written validator.py and res/schema.json had been two independent, drifting definitions of "valid seed data" ever since the jsonschema dependency was dropped in v0.3.0.dev2 (the ! relationship prefix landed in the same commit and broke the schema's object-vs-scalar assumption). This closes four validation gaps and regenerates the schema so both describe the exact same format. Advances #42.

Validator fixes

• Empty entity {} — previously passed validation, then crashed the seeder with an opaque AttributeError. Now raises a clean MissingKeyError. (The len == 0 early-return that skipped the model check is gone; empty lists are still a valid "seed nothing" no-op.)
• Closed key set (check_keys) — unknown/typo keys within the old 2-key budget were silently ignored. Now raise InvalidKeyError with a precise message. This also improves the basic Seeder's error for a filter key from the misleading "Missing data key(s)" to "Unexpected key(s): filter".
• Exactly one source key — an entity with both data and filter silently used data. Now raises InvalidKeyError.
• Empty data: {} vs data: [] — kept as-is by design ({} = one row of DB defaults, [] = zero rows). Documented rather than changed.

Schema realignment

• res/schema.json regenerated to mirror the fixed validator: ! prefix via patternProperties, additionalProperties: false (closed key set), oneOf (exactly one source), and a parent_entity/entity split encoding the parent-requires-model / child-optional rule (the distinction the old parent.json once needed a whole second file for).
• Cross-validated against all 32 test fixtures: 0 mismatches between the schema and hybrid_validate.
• Added package-data so the schema ships in the wheel (verified), and a README section on wiring it into editors (YAML modeline + VS Code json.schemas/yaml.schemas).

Cleanup

• Removed the now-unused MaxLengthExceededError (not part of the public API / docs).

Test plan

• 6 new validator tests (empty dict parent/child, unknown key parent/child, both-sources, basic-rejects-filter); updated the former extra-key test to expect the more precise InvalidKeyError.
• Full suite: 98 passed.
• Schema is well-formed Draft-07 and agrees with the validator on every fixture.

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