site: reduce full Pages build latency

[TankTechnology]

Evidence

The previous successful Pages run took approximately 2 hours 27 minutes; the lake build :literate preparation step accounted for approximately 2 hours 2 minutes.

Boundary

Keep immutable Literate JSON inputs and the sharded renderer while making source-local changes reuse valid work across commits.

Acceptance

• Cache identity contains every input that affects each reusable unit.
• A source-local change demonstrates a materially shorter successful Pages run.
• Fresh-cache behavior renders and validates every expected page.