fix(parser): prev of binds unary-or-tighter, rejects non-name operand (#634)

[InauguralPhysicist]

Closes #634

prev of was the only of in the language that didn't bind unary-or-tighter. The operand parser called parse_expression where every other of uses parse_unary, so prev of x + 1 parsed as prev of (x + 1) and silently returned null — the whole trailing expression was swallowed.

x is 10
x is 20
prev of x          # 10
(prev of x) + 1    # 11
prev of x + 1      # null   <- should be 11

It's now (prev of x) + 1, matching SPEC.md's Application rule (item 2: "of binds tighter than arithmetic: f of x + 1 is (f of x) + 1") — which prev was the lone exception to. Every occurrence in the suite was written parenthesized, so the form the spec's own rule requires was never tested — textbook "works only because the examples are too easy."

Secondary silent-tolerance

The parser's own comment says "only IDENT operands are meaningful", yet a non-name operand returned null rather than erroring. prev looks back through a variable's assignment history — a literal, index/dot, or parenthesised expression has no trajectory. It's now a clean parse error, so the program doesn't run instead of propagating a null:

$ echo 'x is 5; r is prev of (x + 1)' | eigenscript
Parse error line 1: 'prev of' requires a variable name

Interactions verified unchanged

This was the design-sensitive part — prev is a soft keyword with special forms:

• at suffix: prev of x at L still works (the suffix is parsed after the operand).
• identifier fallback: bare prev as a variable with postfix — prev[0], prev.k — untouched (different parse branch).
• test_temporal.eigs and test_soft_keyword_idents.eigs both still pass.

(parse_unary needed a forward declaration before parse_primary.)

Tests

• test_soft_keyword_idents.eigs SK19/b/c: the unparenthesized +/*/- forms — each returned null before the fix.
• Runner [70b]: prev of (x+1) is a parse error and the program doesn't run.
• SPEC.md's temporal section documents the operand rule (prose, outside the byte-checked example blocks).

Also backfills the [Unreleased] Fixed CHANGELOG entries for #630, #632, #631, which merged without them.

Gates

• Release suite 3086/3086
• ASan + UBSan detect_leaks=1 3090/3090, leak tally 0
• Doc examples 71/71, doc-drift clean

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