Implement Set-based coverage tracking

[tothtamas28]

Replaces the previous Bytes-based approach to coverage tracking with a structured Set-based representation. Coverage is now stored as a Map of #coverage(size, entries) values rather than raw Bytes arrays, where entries is a K Set of #covEntry(offset, length) records.

Motivation

The Set representation avoids copying a large byte array on each update (including idempotent ones), and accumulates entries in a persistent set instead.

Changes

K semantics: coverage.md

• Add CovEntry and Coverage syntax productions
• #initCoverage initializes accounts with #coverage(size, .Set) instead of zero-padded Bytes
• #updateCoverage accumulates entries via #addEntry, inserting into the Set with |Set SetItem(ENTRY)

Rust FFI: skribe-fuzz-rs/src/lib.rs

• Add match_coverage / match_cov_entry to deserialize the new K pattern format
• Add match_set / collect_set_entries to flatten K's left-associative Set
• write_coverage_data expands entries into 0xff byte ranges, preserving the fuzzer bitmap interface
• Extract shared match_dv / match_symbol / match_app helpers
• Fix get_cell to return the matched node rather than its first child