add local package dependencies

[kacy]

summary

Adds the first local-package workflow for Pith:

• resolves local path dependencies from pith.toml and pith.lock
• adds package commands for check/test/lint/doc/deps/lock/install/inspect/context
• installs local path dependencies into an ignored .pith/packages cache
• hardens install with recursive in-process copying, cache segment validation, explicit CLI errors, and --refresh
• adds a small package fixture covering dependency imports

tested

• ./self-host/pith_main package install
• ./self-host/pith_main package install --refresh
• ./self-host/pith_main package install --bad
• ./self-host/pith_main package deps
• ./self-host/pith_main package check
• ./self-host/pith_main package test
• ./target/release/pith run tests/package_deps/app/lib.pith
• temporary out-of-tree package fixtures for missing dependency path, missing version, invalid dependency name, invalid dependency version, and dependency paths with spaces
• ./self-host/pith_main check self-host/pith_main.pith
• ./self-host/pith_main fmt --check self-host/pith_main.pith
• make self-host
• make check-no-panics
• cargo test --manifest-path cranelift/Cargo.toml --workspace --locked

design notes

This remains local-first. Remote registry semantics, checksums, transitive dependency install, and stronger manifest parsing are left for follow-up commits.