fix: preserve sparse overlay intent across merges

[mikehostetler]

Summary

• preserve sparse build-time overlays until after merge so partial source records do not erase lower-precedence data
• force local TOML sources to the highest precedence tier regardless of configured source order
• deep-merge sparse runtime custom provider/model overlays and revalidate the merged result instead of replacing packaged records
• regenerate the installed snapshot and add regression coverage for sparse capability overlays, local precedence, and runtime custom overlays

Callouts

• Build-time engine fix: source layers now stay sparse through merge, so overlay intent is preserved and local TOML overrides remain absolute.
• Runtime loader fix: LLMDB.load(custom: ...) now behaves like a sparse overlay instead of a full record replacement.

Closes #173