actiondfs: simplify caching, inode ownership, and staged directories

[dzbarsky]

Summary

• Remove 381 net lines from actiondfs.c across ten independent simplification commits.
• Remove duplicated staged-directory and mount state, consolidate protobuf parsing and child validation, and simplify inode ownership, mount options, cache cleanup, and staged I/O.
• Preserve digest validation at mount/protobuf ingress while removing approximately 40 million redundant hexadecimal-character checks from the LLVM workload.

Validation

• Remote Bazel build: 66 repository targets, including ARM64/x86-64 kernels and both production/instrumented actiondfs configurations; excludes 12 generated kernel host-tool targets that already fail as standalone macOS targets on main.
• Remote Bazel tests: //src:unit_tests and //tools:e2e_action_tool_tests.
• Standalone timing-parser regression test: //:parse_timings_test.
• Production and instrumented tools/e2e.sh vm on macOS Virtualization.framework.
• Fresh-worker LLVM VM smoke: 2,017 warmup actions and 2,106 measured actions; measured build improved from 138.481s to 108.831s (21.4%), with zero blob-path-cache insertion races and zero staged-operation failures.