Split production and experimental ForkPress builds

[JanJakes]

Summary

This PR separates the shippable forkpress binary from experimental storage work:

• keeps production focused on materialized COW storage with APFS clonefile, APFS sparsebundle, Linux FICLONE reflinks, and file-copy fallback
• adds a separate forkpress-dev binary behind the dev-experiments Cargo feature for BranchFS, CAS, and ZFS experiments
• splits production Rust into focused crates under crates/: forkpress-cli, forkpress-core, forkpress-storage, forkpress-runtime, forkpress-server, and forkpress-git
• keeps experiment Rust crates with their experiment code, currently experiments/cas/crates/{cas-store,cas-ffi}
• keeps production runtime inputs narrow under runtime/cow/, scripts/cow/, scripts/git/, and scripts/shared/
• groups BranchFS runtime, scripts, Git adapter, PHP extension, schema, and tests under experiments/branchfs/
• groups CAS runtime, Rust crates, and e2e coverage under experiments/cas/
• moves ZFS code under experiments/zfs-engine/
• updates CI, release jobs, Makefile targets, README, storage docs, and contributor docs for the two-binary and multi-crate layout
• includes COW Git hardening from this branch, including signed commit parsing, branch mutation locking, Git-created branch support, branch deletion/reset, and path normalization checks

Stacking note: this work was developed on top of codex/cow-database-sql-schema-snapshot-clean and is now rebased onto latest trunk.

Binary size

Measured on macOS arm64 release builds with the embedded runtime bundle:

Build	Size	Delta vs pre-split single-binary base
before: single forkpress from the pre-split base	36.11 MiB / 37,867,648 bytes	baseline
after: production forkpress	34.24 MiB / 35,906,592 bytes	-1.87 MiB / -5.2%
after: forkpress-dev with experiments	36.13 MiB / 37,885,328 bytes	+0.02 MiB / +0.0%

The production PHP runtime itself drops from 25.78 MiB to 23.38 MiB (-2.40 MiB / -9.3%) because BranchFS/CAS are no longer built into production PHP. The dev PHP runtime remains at 25.78 MiB, matching the previous all-experiments shape. The end-to-end binary delta is modest because the embedded WordPress/PHP/runtime assets dominate the final binary size.

Validation

• cargo test --workspace --exclude forkpress-cli
• cargo test -p forkpress-core --features dev-experiments
• FORKPRESS_RUNTIME_BUNDLE=/dev/null cargo test -p forkpress-cli --bin forkpress
• FORKPRESS_RUNTIME_BUNDLE=/dev/null cargo test -p forkpress-cli --features dev-experiments --bin forkpress-dev
• cargo build --release -p forkpress-cli --bin forkpress
• cargo build --release -p forkpress-cli --features dev-experiments --bin forkpress-dev
• cargo fmt --check
• git diff --check
• bash -n scripts/build-dist.sh
• bash -n scripts/release/try-single-binary-detach.sh
• bash -n tests/cow/e2e.sh
• bash -n experiments/cas/tests/e2e.sh
• make -n forkpress forkpress-dev test-all
• PHP lint over scripts/, runtime/cow/, experiments/branchfs/, experiments/cas/runtime/, tests/cow/, and experiments/branchfs/tests/
• make test-cow
• make test-branchfs
• target/release/forkpress --help
• target/release/forkpress-dev --help
• inspected embedded runtime tarballs: production contains no experiments/**; dev contains BranchFS/CAS experiment runtime and scripts

[JanJakes]

I did a couple of review rounds, and all seems OK, so I'll get it in now.