actiondfs: simplify child parsing, staged mutations, and inode identity

[dzbarsky]

Five focused commits simplify shared REAPI child parsing; staged file/symlink creation; staged unlink/rmdir; backing opens and hexadecimal parsing; and immutable child identity, symlink ownership, and staged-directory ancestry. Removes 149 net lines from kernel/actiondfs/actiondfs.c and eight bytes per input node.

Validation: full ARM64/x86_64 production and instrumented kernel builds, bazel build/test //..., the standalone timing-parser test, production and instrumented tools/e2e.sh vm, and two fresh-worker LLVM smokes with identical 2,017 warmup plus 2,106 measured actions. Local LLVM times were distorted by severe host CPU/memory contention: 137.064/151.585 seconds and 166.200/196.180 seconds versus the prior 102.136/108.831 seconds; identical action digests and filesystem counters isolate the slowdown to action process scheduling.

All five GitHub Actions checks pass. The 1,998-action Linux comparison measured 275.435 seconds for actiond and 263.800 seconds natively (1.044×); the main baseline was 301.049/298.626 seconds (1.008×). The 1,998-action Windows comparison measured 355.531 seconds for actiond and 362.304 seconds natively (0.981×); the main baseline was 370.627/392.874 seconds (0.943×). The normalized ratios are approximately 3.6% slower on Linux and 4.0% slower on Windows. The Linux check initially reproduced the intermittent keepalive stall previously observed on main, then passed when only the failed job was rerun.