price the read-seam wrapper depth bound, and pin it from above (#13472)

The bound was documented from the day it was written; what it COSTS never
was. Measured on 71627f7: 6 real read seams, all admitted at depth 3,
unchanged to depth 50, all six carrying `no invented answer` — so the
bound holds a denominator still, it does not buy a finding.

Before this change the constant was constrained from BELOW only: lowering
it to 1 reddens one fixture, raising it to 3/4/6/50 left all 51 read-seam
fixtures and every gate in this file green while the census moved 66 to
72. Two fixtures now pin it from above, and `--depth-cost` re-derives the
price on demand so it cannot go stale in a comment again.

Zero-delta: the full `--list` output is byte-identical before and after.


Claude-Session: https://claude.ai/code/session_01Pk26oZ12t5N1hwGW1m1MgC

Co-authored-by: Claude <noreply@anthropic.com>

Author: os-project-manager