fix: remove validation on reads, matching documented behavior

[brainkim]

Summary

• Remove validateWithStandardSchema call during reads in buildEntityMap
• Update tests to use z.date()/z.boolean()/z.number() instead of z.coerce.*
• Zen's decodeData handles DB→JS type conversion (dates, booleans, JSON)
• Zod coerce/transform only runs on writes, not reads

Background

The README states "Zod validation happens on writes, never on reads" but validation was running during normalize() via buildEntityMap(). This caused ValidationError when reading data written with an older schema version.

Data flow:

• Writes: App → Zod parse (validate + coerce/transform) → Zen encode → DB
• Reads: DB → Zen decode → App (no Zod)

Zen's decodeData handles all DB→JS conversions:

• Date strings → Date objects
• 0/1 → booleans
• JSON strings → objects/arrays

Users should use z.date() not z.coerce.date() — Zen handles the conversion.

Test plan

• All 584 tests pass
• Type checking passes

Fixes #14

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