Add pup show index forensic command

[typeless]

Summary

Adds pup show index — a forensic command that dumps the on-disk .pup/index in a grep-friendly form. The intended use is to answer "did pup record this implicit-dep edge?" — the core diagnostic question for the transitive-dep tracking bug under investigation (see pup-header-detection-bug.md).

Usage

pup show index                  # full dump: per-command implicit/sticky edges
pup show index --summary        # counts only + "Commands with implicit/sticky deps: N/M"
pup show index PATTERN          # full dump filtered to commands whose command-string contains PATTERN

Example output:

[build] Index: /home/mural/src/pup/build/.pup/index
  Files:    832
  Commands: 101
  Edges:    39201 (Normal=352, Sticky=795, Implicit=38054)

Commands (with implicit/sticky edges):
  c2147483649  [test/unit]
       cmd: g++ -std=c++20 ... -MD -c .../catch_amalgamated.cpp -o ...
       implicit: third_party/catch_amalgamated.cpp
       implicit: /usr/include/stdc-predef.h
       ...
       sticky:   configs/Tupfile

Implementation

• New cmd_export_index in src/cli/cmd_show.cpp dispatched from the existing show <format> mechanism. Joins script, compdb, graph, var, instructions as the sixth format.
• Loads the index directly via discover_layout + read_index — does not parse Tupfiles. The forensic remains usable when the build graph is in a partially-broken state.
• Extends options.cpp so show index PATTERN parses the positional argument into show_var_filter (same slot show var NAME uses).
• Help text in print_usage() updated.

Test plan

• New [e2e][show] SCENARIO (Show index dumps implicit-dep edges from the on-disk index) covers default output, --summary mode, and the positional filter
• Manual smoke test against pup's own build/ directory: 832 files, 101 commands, 38054 implicit edges, all commands list their -MD-discovered headers
• Full test suite: 462 cases, 23,644 assertions (one new SCENARIO, 22 new assertions)
• make format clean

Once pup-header-detection-bug.md's reproducer is merged (PR #48), this command can be run on the broken state — e.g. pup show index PATTERN, look for newhdr.h in the implicit list — to confirm whether the dep was ever recorded.

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