Add experimental Fleet runtime orchestration API

[jezdez]

Summary

• keeps stamped runtime binaries and automatic first-run bootstrap as the primary conda-ship workflow
• adds an opt-in conda_ship::fleet Rust API for downstream orchestrators that manage several locked prefixes
• reuses the process lock, bootstrap state, metadata, bundle handling, and reinstall code used by stamped runtimes
• requires an explicit delegate for every Fleet runtime and returns executable and PATH data
• leaves condarc content, base freezing, and Constructor installer metadata disabled unless the caller requests them
• leaves launcher update and removal policy with the Fleet caller

Compatibility boundaries

• Fleet adds no runtime command namespace and does not intercept delegate arguments
• Fleet has no fallback to conda and does not set conda activation variables
• lockfile channels are recorded as provenance and never become an implicit condarc
• force performs an in-place full-lock reinstall while preserving named environments and unrelated files
• recursive deletion remains an explicit Fleet::remove operation under the same adjacent prefix lock
• ready metadata is written only after package installation, configured condarc and CEP 22 files, Constructor metadata, bytecode compilation, and delegate validation succeed
• Fleet rejects invalid specs, foreign metadata, symlinks, and nonregular managed files before installing packages
• stamped templates remain unaffected unless the non-default fleet Cargo feature is enabled

Launcher ownership

Fleet does not create or consume direct-install launcher receipts. The optional receipt API in #73 is independent and is not a prerequisite.

Follow-ups

• Adapt stamped runtime artifacts for Fleet #46 adapts stamped runtime artifact data into RuntimeSpec
• Test Fleet with stamped runtime artifacts #51 adds end-to-end coverage using actual stamped artifacts

Closes #48
Closes #49
Closes #52

[jezdez]

@mattkram I restacked and narrowed #45. Fleet remains an optional API in conda-ship. It installs, lists, gets, and removes locked prefixes, then returns the executable and PATH entries. It does not create wrappers, manage PATH, or own product update policy. Those remain ana's responsibility.