Chapter 25: add separate compilation and simplify SSA construction

Introduce Chapter 25 with separately compiled Simple modules, serialized IR,
cross-compilation-unit linking, and native system-library integration.  This
also completes a broad parser and type-system cleanup needed to make graph
construction independent of incomplete parse-time type information.

Separate compilation and linking

- Add compilation-unit Start/Stop nodes and track functions, classes, aliases,
  and global indices across compilation-unit boundaries.
- Serialize canonical types and ideal graph nodes into ELF objects, including
  dependency metadata, relocations, global/local bitset remapping, and dense
  memory-alias information.
- Load dependent objects on demand, recursively resolve source and object
  paths, relink calls and function pointers, and preserve external C symbols.
- Distinguish the post-optimization ownership hook from unknown callers so
  private call graphs can remain precise while public functions remain
  externally callable.
- Add command-line and build support for compiling modules and the reusable
  `sys` object library.

Parser, constructors, and SSA

- Replace inline constructor expansion with a hidden default `<init>` followed
  by an explicit user constructor declared as `new Type = { ... }`.
- Keep the default constructor private, pass private object memory through
  constructor calls, and require user constructors to initialize non-default
  fields before the object escapes.
- Track possibly-uninitialized fields in parser `Var` state and diagnose early
  reads without encoding parser-only constraints in Load and Store nodes.
- Make ordinary parser memory bulk and recover precise alias slices in the
  optimizer using `MemMerge`, `MemPhi`, and `BulkMemPhi` nodes.
- Remove parser-supplied type constraints from Loads and ordinary Phis, defer
  integer/float arithmetic selection, and keep call shape driven by syntax
  instead of provisional inferred types.
- Preserve chained field declarations as static parser information when flow
  types are not yet available, including postfix field updates through calls.

Types, functions, and escape analysis

- Add `TypeScalar` as the common lattice family for integer, float, memory
  pointer, and function-pointer values, with scalar TOP/BOT values for generic
  Loads, Stores, parameters, and invalid intermediate expressions.
- Represent unresolved struct definitions directly with the `TypeStruct`
  forward-reference flag while retaining the separate open-structure concept;
  close and canonicalize recursive structures without side maps.
- Add explicit `FunPtrNode` references so constant function pointers retain a
  graph edge to their Return, and keep referenced functions alive until Opto
  proves the pointer dead.
- Refine public-function and class-initializer escape handling, including
  public-only fallback escape sets and monotone compilation-unit summaries.
- Add guards, proof casts, read-only propagation, pointer-to-integer conversion
  for non-null FFI arguments, and consistent deep mutability checks at calls.

Optimization and code generation

- Make weak and unresolved graph inputs compute conservatively and monotonically
  throughout iterative peepholes and SCCP.
- Preserve independent memory slices when sharpening Stores, retain control on
  conditional Stores, and freeze Store widths from their target field types.
- Extend serialization for new node kinds, scalar types, Store modes, function
  pointers, and memory partitions, preserving serialize/deserialize bijection.
- Complete native selection and encoding support across x86-64, RISC-V, and
  ARM, including FFI pointer conversion and missing integer operations.
- Fix x86 immediate multiply REX encoding and assorted scheduling, register
  allocation, spilling, relocation, and constant-pool issues exposed by the
  larger linked graphs.

System library, tests, and documentation

- Compile and link the Simple `sys` library, including libc bindings, I/O,
  scanners, strings, vectors, array helpers, and executable entry points.
- Add Chapter 25 module, serialization, constructor, forward-reference,
  postfix-update, system, Hello World, and Bubble Sort coverage; update golden
  graphs and spill expectations for the new IR shape.
- Add module and type-system documentation, parser-simplification design notes,
  grammar updates, examples, diagrams, and graph visualization assets.

The resulting compiler passes the raw Chapter 1-24 suite and the Chapter 25
system and separate-compilation tests, including a freshly rebuilt `sys.o` and
the native Bubble Sort example.

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