32-bit recoder for trace files

[digama0]

Summary

• Add a streaming recode32 binary that converts PolyML's 64-bit .tr heap dumps to a 32-bit narrow format, halving the size of pointers and headers
• The 32-bit variant is identified by an 8-byte magic TR32\0\0\0\xFF; the final byte cannot collide with a valid 64-bit flags byte (which is always 00–03)
• ProofTraceParser gains a Heap32 variant that decodes the narrow format in-place; all sh* accessors dispatch on the variant
• Tracing.export pipes through recode32 when the (default-on) recode_32bit flag is set
• The recoder raises Fail rather than silently truncating when a value does not fit in 32 bits
• recode32 is built automatically by Holmake (via POLYC) into $HOLDIR/bin/recode32 when --trknl is enabled, since src/tracing/yes is in the kernel sequence for that build

Test plan

• Build with --trknl and confirm $HOLDIR/bin/recode32 is produced
• Verify .tr.gz files are roughly half the size of the 64-bit format
• Confirm ProofTraceParser.parse decodes the new files correctly via ProofTraceReplay