Fix invalid TOML from commented multi-line values (#1081)

* Fix invalid TOML from commented multi-line values

A struct field marked `commented` that renders across multiple lines — a
multiline string or a multiline array (via the `multiline` tag or
SetArraysMultiline) — only had its first line prefixed with the `#`
comment marker. The continuation lines were emitted as live TOML and
failed to parse, e.g.:

    # s = """
    line1
    line2"""

encodeKeyValue now prefixes every physical line of a commented value,
guarded by `if commented` so the common path is unchanged (allocs/op and
B/op are byte-identical in the Marshal benchmarks).

This was surfaced by a generative validity oracle (random struct types,
tags, options and values) that re-parses every marshaled output: the
multiline-string variant produced invalid TOML in every release back to
v2.3.x; the multiline-array variant regressed in v2.4 as more multiline
forms began to be honored.

Also restores the v2.3 layout for `commented` combined with
SetIndentTables (the comment marker stays at column zero, with the table
indentation inside the comment), and adds audit_flags_test.go: an
exhaustive regression suite for the encoder option x struct-tag matrix
and the decoder behaviors reworked by the v2.4.0 reimplementation,
including guards for the #1075 and #1079 fixes.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* Use promoted field access in embedded-tag test (staticcheck QF1008)

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: pelletier