Refactor: [H1] separate HBG host build from device upload

[TaoZQY]

Host graph construction currently performs device allocation and H2D as part of the same operation. Expose a leased Host build result so kernel-mode consumers can inspect and serialize the graph before submitting device work.

Program execution explicitly calls the upload path after building. Upload compacts from the virtual-address source and rebinds retained Definition staging after growth, preserving valid sources across repeated uploads and failed-copy retries.

Validation

• Both HBG C++ suites passed (9 cases per architecture), together with context and contract tests at this milestone.
• Full rebased stack at H3 Add: [H3] seal and validate HBG execution slots #2175: 147 C++ test executables and 159 Python tests passed (4 hardware-marked skips), covering ABI, task interfaces and simulator DMA workspace.
• Full-stack simulation with the pinned PTO ISA required: A2/A3 74 passed, 8 skipped; A5 71 passed.
• Runtime libraries rebuilt; repository pre-commit checks passed. Each earlier milestone was independently compiled and tested.
• mkdocs build --strict passed separately at all five PR heads. The HBG contract page is included in navigation from H1; the execution-slot page is included from H3.

This provides the Host build/upload boundary. It does not enable the public kernel launch or ACLGraph capture/replay path.

Dependency and review scope

Draft 1/5: H1. Depends on K1 #2064.

• Original stage commit (before squash): fa819996.
• Isolated implementation diff.
• Review order: K1 Add: kernel-mode C ABI skeleton, state machine, and wire headers (K1) #2064 → H1 Refactor: [H1] separate HBG host build from device upload #2171 → 2B Add: [2B] declare HBG resource and stream contracts #2172 → context prepare/freeze Add: [Context Prepare] prepare and freeze kernel resources #2173 → H2 Add: [H2] serialize immutable HBG packets for HostArgs #2174 → H3 Add: [H3] seal and validate HBG execution slots #2175.

This draft is rebased onto main at a1aa7fddf42b2d28849bc3fb6fe1eb50ab891324 and squashed to one commit. The rebase retains the upstream documentation-theme update. Source and test files match the previous PR head exactly; the squash preserves the complete rebased tree. Applicable pre-commit checks passed again.

All five HBG drafts target main. Each PR has exactly one commit, containing K1 plus all HBG milestones through the stage in its title. Squashing changes commit history, not the cumulative code scope. The original isolated diff above remains available to review this stage alone. Rebase in dependency order after prerequisites land.

The K1 snapshot originally carried from 21fb15c0 is included in this squashed commit, with its human author retained as co-author. Upstream #2064 remains open and has advanced; alignment with its final version remains public execution integration work.

No idle onboard devices were available. Simulation golden checks cover existing program-mode regression; mixed ACLGraph capture/replay, DSV4 CSA device accuracy and comparative performance remain unvalidated.

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