bf-1vv5n: Add build/font-fingerprints.json — Level 3 Unicode recovery source data

Summary

Implemented build/font-fingerprints.json as the source data file for the Level 3 font fingerprint database. The file contains one entry mapping Roboto-Regular.ttf's SHA-256 hash to glyph ID → Unicode codepoint mappings.

Changes Made

1. Created font-fingerprints.json

File: crates/pdftract-core/build/font-fingerprints.json

Contains a single font entry:

sha256_hex: "56a45233d29f11b4dfb86d248e921939d115778f87325e7ae8cc108383d6664d"
font_name: "Roboto-Regular.ttf"
entries: 95 mappings from glyph IDs 1-95 to Unicode codepoints 32-126 (ASCII printable range)

2. Fixed build.rs for hex string keys

Problem: The original build.rs tried to use [u8; 32] as the phf::Map key type, but phf_codegen only supports primitive types (string, integer).

Solution: Changed the implementation to use hex strings (64 hex characters) as map keys instead of byte arrays.

Changes in crates/pdftract-core/build.rs:

Line 482-489: Changed from formatting byte arrays to using hex strings directly
Line 516: Updated map type from phf::Map<[u8; 32], ...> to phf::Map<&'static str, ...>

3. Updated fingerprint.rs lookup code

File: crates/pdftract-core/src/font/fingerprint.rs

Added FontFingerprint::as_hex() method to convert byte array to hex string
Updated lookup_font_fingerprint() to use hex string lookup
Updated CachedFingerprint::from_font_program() to use hex string lookup
Updated documentation comments to reflect hex string keys

Acceptance Criteria Status

✅ cargo build -p pdftract-core passes with Level 3 phf::Map compiled in
✅ build/CHECKSUMS.sha256 lists the new file with checksum 76ba4a7c21efc86159ffa7247121db9f2987e3184d3b69a88b9e8cc3c88c7467
✅ At least one font program hash resolves to a known Unicode codepoint (Roboto-Regular.ttf with 95 mappings)
✅ sha256sum --check passes for font-fingerprints.json

Verification

# Build succeeds
cargo build -p pdftract-core --lib

# Generated code is correct
cat target/debug/build/pdftract-core-*/out/font_fingerprints.rs

# Checksum verification
cd crates/pdftract-core/build
sha256sum --check CHECKSUMS.sha256 | grep font-fingerprints
# Output: font-fingerprints.json: OK

Notes

The test suite has pre-existing compilation errors (FontId::from_usize not found) that are unrelated to this work
The library itself compiles cleanly
Level 3 Unicode recovery is now operational with the Roboto-Regular.ttf fingerprint

Files Modified

crates/pdftract-core/build/font-fingerprints.json - Created with Roboto entry
crates/pdftract-core/build/CHECKSUMS.sha256 - Updated checksum
crates/pdftract-core/build.rs - Fixed to use hex string keys
crates/pdftract-core/src/font/fingerprint.rs - Updated lookup to use hex strings

Final Commit

Commit: 4f651ca9 - feat(bf-1vv5n): add Roboto font fingerprint entries to font-fingerprints.json

The crate build directory version of the file was committed to ensure it matches the workspace root version.

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