reader: lazy index reads (Prelude + SparseReader) for WMTS-scale packs

[jacovdbergh]

What

A second reading discipline for indexes too large to fetch whole. A WMTS-style zoom level packed as one single-level planar tilepack runs to millions of tiles, so the offset index alone reaches tens of MB — more than a serving process wants to hold (or fetch) per open pack. It doesn't need to:

• Prelude — parses header + descriptors only (~100 bytes; Truncated-staged exactly like FrontMatter::parse, needing at most descriptors_end). Then:
  • index_entry_span(ordinal) → the 16-byte file span holding a tile's start/end offsets (24 + 48g + 8i);
  • tile_range_from_entry(&[u8; 16], file_len) → the blob range, applying the per-entry half of the whole-index validation (backwards ranges, ranges into the front matter, and ranges past a known file length are BadIndex; equal offsets = absent tile, Ok(None)).
• SparseReader<R: Read + Seek> — the I/O twin: prelude up front, then per tile one 16-byte index read + one blob read. O(1) memory regardless of index size.

No wire-format change, no new deps, wasm-clean. FrontMatter stays the right choice whenever the index fits (panorama viewers untouched).

Tests

• The roundtrip property test (400 cases, arbitrary layouts/groups/absences) now also asserts, for every ordinal: Prelude entry arithmetic + validation ≡ FrontMatter::ordinal_range, and SparseReader ≡ the eager view. The lazy path can't drift from the eager one without the oracle catching it.
• tests/lazy.rs: golden-fixture equivalence (sparse ≡ eager for all ordinals), staged Truncated needs, and adversarial entry rejection (backwards / front-matter overlap / past-EOF, plus the documented "unknown file length ⇒ oversize is the caller's to catch" case).

Docs

• SPEC.md: informative paragraph in HTTP access pattern — the per-tile 16-byte index read and the validation a lazy client MUST do instead of the whole-index pass.
• docs/integration.md: new Lazy index reads section for the WMTS-scale consumer.

All four gates pass: cargo test --workspace, clippy --all-targets -D warnings, fmt --check, wasm32 release build of -p tilepack.