bug(cache): retain MaterializedLnk leases during ESP32 embed processing

[zackees]

Problem

The ESP32 embed-processing path extracts a filesystem path from each MaterializedLnk cache object, but drops the lease object immediately. _lnk_leases is declared for lifetime retention yet the created lease is not appended, so cache cleanup/eviction can invalidate a materialized .lnk path while the consuming build step still expects it to exist.

Source

Unresolved CodeRabbit thread on merged PR #119:
#119 (comment)

A current-main audit found no duplicate issue and confirmed the retention gap remains.

Acceptance criteria

• Retain every MaterializedLnk in _lnk_leases for the full ESP32 embed-processing operation.
• Add a regression that proves the materialized cache lease remains alive while its extracted path is consumed.
• Release leases normally when the operation ends.