docs(proposal): add iterative bootstrap proposal

docs/proposals/index.rst

@@ -4,6 +4,7 @@ Fromager Enhancement Proposals
 .. toctree::
    :maxdepth: 1
 
+   iterative-bootstrap
    new-patcher-config
    new-resolver-config
    release-cooldown

docs/proposals/iterative-bootstrap.rst

@@ -0,0 +1,60 @@
+Proposal: Convert Bootstrapper from Recursive to Iterative
+==========================================================
+
+Problem
+-------
+
+The ``Bootstrapper`` class processes dependency trees using recursive calls.
+When bootstrapping packages with deep or wide dependency graphs — especially
+with ``--multiple-versions`` enabled — this hits Python's recursion depth limit
+and causes stack overflow errors. The recursion occurs at two points: processing
+install dependencies after building a package, and processing build
+dependencies before building.
+
+Proposed Solution
+-----------------
+
+Replace the recursive depth-first traversal with an iterative approach using an
+explicit work stack. Each package version to bootstrap is represented as a
+``BootstrapWorkItem`` dataclass that flows through five linear phases:
+
+1. **START** — Add to dependency graph and check if already seen (early exit if so)
+2. **EXTRACT_BUILD_DEPS** — Collect build dependencies and push them onto the stack
+3. **BUILD_PACKAGE** — Build sdist and/or wheel, record in build order
+4. **EXTRACT_INSTALL_DEPS** — Extract install dependencies and push onto the stack
+5. **COMPLETE** — Clean up build directories
+
+The LIFO stack ensures depth-first traversal order is preserved. Build
+dependencies are pushed in reverse order so they pop in the correct sequence
+(BUILD_SYSTEM before BUILD_BACKEND before BUILD_SDIST). Each dependency
+completes all five phases before its parent continues to the next phase.
+
+This pattern already exists in the codebase: ``dependency_graph.py`` and
+``commands/graph.py`` both use stack-based DFS traversals.
+
+Scope
+-----
+
+This is a pure refactoring — no behavior changes. The external interface
+(``bootstrap()`` method signature, CLI flags, output files) remains identical.
+All existing error handling modes (normal fail-fast, ``--test-mode``,
+``--multiple-versions``) are preserved. Progress bar semantics are unchanged.
+
+The change is limited to ``src/fromager/bootstrapper.py``. No other files
+require modification. All existing tests must pass without changes.
+
+Benefits
+--------
+
+- Eliminates recursion depth limits entirely (work stack lives on the heap)
+- Handles arbitrarily deep and wide dependency graphs
+- Improves debuggability with explicit state in work items
+- Follows proven patterns already established in the codebase
+
+Verification
+------------
+
+- All existing unit and e2e tests must pass unchanged
+- Bootstrap output for the same inputs must be identical
+- Performance should remain within 10% of the recursive version
+- Rollback via ``git revert`` if issues are discovered post-merge