v3: add eval backend

[medvednikov]

Summary

• add v3.eval, a flat-AST interpreter with the v2 eval-style public API (new, create, run_text, run_files, stdout, stderr)
• wire v3 -b eval to run the evaluator after parse/type-check and before transform/codegen
• add focused evaluator tests for direct API execution and the v3 CLI eval backend
• keep string slicing addressable for -gc none -prod builds
• address eval review findings across multi-return assignment, flow propagation, scoping, typed adaptation, enum/map/array/struct behavior, defers, closures, imported-module context, builtins, top-level comptime declarations, global declared type lookup, implicit-main execution, match type-pattern disambiguation, single-evaluation array append lvalues, sequential value-block results, typed append RHS evaluation, mutating method receiver writeback, enum casts, array initializer semantics, if-guard map/option semantics, and append statements inside value blocks

Latest Review Pass

• pushed 123b547dc for the latest review comment: value-block append expression statements now execute through the statement executor before the final block value is produced
• resolved the covered review thread; the current unresolved review-thread count is 0

Tests

• ./v -g -keepc -o ./vnew cmd/v
• ./vnew test vlib/v3/eval/eval_test.v
• ./vnew -check vlib/v3/v3.v
• git diff --check -- vlib/v3/eval/eval.v vlib/v3/eval/eval_test.v
• from vlib/v3: ../../vnew -gc none -prod -o /tmp/v3_prod_nogc v3.v

[medvednikov]

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[medvednikov]

Follow-up pushed in cfb6c6e for the latest eval review:

• implicit-main top-level statements are recorded and executed when no explicit main.main exists
• match type-pattern checks now run only for TypeValue conditions, so value patterns like "int" stay value patterns

Validation run locally:

• ./vnew test vlib/v3/eval/eval_test.v
• ./vnew -check vlib/v3/v3.v
• cd vlib/v3 && ../../vnew -gc none -prod -o /tmp/v3_prod_nogc v3.v
• /tmp/v3_prod_nogc -b eval <implicit-main smoke file>

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[medvednikov]

Follow-up pushed in de5950a for the latest eval review:

• exec_array_append now resolves the append target once and writes back through the resolved lvalue path
• value blocks now keep sequential expression statements as side effects plus the last value, while preserving nested comma-expression blocks for multi-return values

Validation run locally:

• ./v -g -keepc -o ./vnew cmd/v
• ./vnew test vlib/v3/eval/eval_test.v
• ./vnew -check vlib/v3/v3.v
• git diff --check -- vlib/v3/eval/eval.v vlib/v3/eval/eval_test.v
• cd vlib/v3 && ../../vnew -gc none -prod -o /tmp/v3_prod_nogc v3.v

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[medvednikov]

Follow-up pushed in 6951af7 for the latest eval review:

• array append now resolves the target first and evaluates the RHS with the array element type when appropriate, so enum shorthands use the element enum
• mutating method receiver writeback now reuses the already-resolved receiver lvalue
• enum casts now return EnumValue for enum targets before sum/fallback handling

Validation run locally:

• ./v -g -keepc -o ./vnew cmd/v
• ./vnew test vlib/v3/eval/eval_test.v
• ./vnew -check vlib/v3/v3.v
• git diff --check -- vlib/v3/eval/eval.v vlib/v3/eval/eval_test.v
• cd vlib/v3 && ../../vnew -gc none -prod -o /tmp/v3_prod_nogc v3.v

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[medvednikov]

Fixed the latest array initializer review points in 8d0a01a88:

• init: expressions are now evaluated per element with index bound, including fixed arrays.
• fixed-array lengths now resolve const identifiers instead of only numeric literals.
• added regressions for per-index init: evaluation and const-sized fixed arrays, including struct zero values.

Validation:

• ./vnew fmt -w vlib/v3/eval/eval.v vlib/v3/eval/eval_test.v
• ./vnew test vlib/v3/eval/eval_test.v
• git diff --check -- vlib/v3/eval/eval.v vlib/v3/eval/eval_test.v
• ./v -g -keepc -o ./vnew cmd/v
• ./vnew test vlib/v3/eval/eval_test.v
• ./vnew -check vlib/v3/v3.v
• cd vlib/v3 && ../../vnew -gc none -prod -o /tmp/v3_prod_nogc v3.v

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[medvednikov]

Fixed the latest if-guard review point in 5ceb871e4:

• map lookup guards now branch on lookup presence instead of default-value truthiness, so present zero values enter the branch and missing keys do not.
• option/result guards bind the unwrapped payload with the inner type instead of rebinding it as ?T/!T.
• added regressions for map lookup guard presence and ?int guard payload binding.

Validation:

• ./vnew fmt -w vlib/v3/eval/eval.v vlib/v3/eval/eval_test.v
• ./vnew test vlib/v3/eval/eval_test.v
• git diff --check -- vlib/v3/eval/eval.v vlib/v3/eval/eval_test.v
• ./v -g -keepc -o ./vnew cmd/v
• ./vnew test vlib/v3/eval/eval_test.v
• ./vnew -check vlib/v3/v3.v
• cd vlib/v3 && ../../vnew -gc none -prod -o /tmp/v3_prod_nogc v3.v

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[medvednikov]

Fixed the latest value-block append review point in 123b547dc:

• value-block expr_stmts that are array append syntax now execute through the statement executor instead of expression evaluation.
• added a regression covering append statements before final values in both if value branches and or blocks.

Validation:

• ./vnew fmt -w vlib/v3/eval/eval.v vlib/v3/eval/eval_test.v
• ./vnew test vlib/v3/eval/eval_test.v
• git diff --check -- vlib/v3/eval/eval.v vlib/v3/eval/eval_test.v
• ./v -g -keepc -o ./vnew cmd/v
• ./vnew test vlib/v3/eval/eval_test.v
• ./vnew -check vlib/v3/v3.v
• cd vlib/v3 && ../../vnew -gc none -prod -o /tmp/v3_prod_nogc v3.v

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