ptx: implement it

[metif12]

ptx produces a permuted index: one row per keyword occurrence with the surrounding context, laid out so every keyword lands at the same column.

The measured layout is the tail right-justified in line_width / 2, then the gap, then the keyword, then one space and the head.

The word is a run of letters by default, not letters and digits — which is what GNU does: 42nd yields the keyword nd with 42 left in the context. My first version included digits, and the differential against GNU caught it.

Verification was by cross-compiling for Linux and running under WSL, since V emits Linux C from Windows but cannot link it.

Three behaviours were wrong on the first attempt and the differential caught all three:

• the keyword was padded, shifting the head one column right on rows with a short keyword
• the head was truncated to the keyword width, cutting beta gamma to beta
• io.read_all stopped early on this platform, producing a single truncated row instead of the whole index

Two known gaps, documented rather than faked:

1. Truncation of a tail or head that does not fit the line width is not implemented. The marker is emitted but the column it lands in does not match GNU, and I did not decode the rule after several attempts. This is the same unresolved thing that made me drop ptx earlier in the session, and it is still unresolved.
2. -r, -A, -t, -M and the -o/-i/-b file filters parse but have no effect.

6 of 10 differential cases match GNU exactly (default, four width/gap geometries, -f, -W, missing-file rc). The 3 truncation cases do not.