[ISSUE-04] Add BuildCompiler API skeleton

[Gonza10V]

Parent: #52
Depends on: #53, #54, #55
Wave: 1

Goal

Add the new public API skeleton for the clean-architecture compiler without implementing real planning/execution behavior yet.

Scope

• Add buildcompiler.api exports.
• Add BuildCompiler with lightweight dependency-injected constructor.
• Add BuildCompiler.from_synbiohub(...) placeholder or minimal factory boundary without constructor-driven indexing.
• Add plan(...), execute(...), and convenience full_build(...) entry points with explicit placeholder behavior where downstream components are not implemented.
• Add import tests.

Non-goals

• Do not implement full planner/executor behavior.
• Do not preserve legacy public signatures.
• Do not import optional automation dependencies from API import paths.

Acceptance criteria

• from buildcompiler.api import BuildCompiler succeeds.
• from buildcompiler.api import full_build succeeds.
• BuildCompiler.__init__ is lightweight and dependency-injected.
• from_synbiohub is the only place reserved for automatic collection indexing.
• Tests assert API imports do not load PUDU/Opentrons/SBOLInventory.

Verification

Run API/domain tests, import smoke tests, and Ruff.

Codex handoff

Keep this skeletal and contract-focused. Real behavior belongs to planner/executor issues.