Accept next inside user-defined functions, unwinding calls at runtime

[bertysentry]

Fixes #580.

Problem

Jawk rejected a next statement inside a user-defined function at compile time (SemanticException: cannot next; not within any input rules), while gawk (including --posix), mawk, and BWK awk accept it and skip to the next input record when the function is called from an input rule.

Change

Mirrors the existing nextfile design:

• AwkParser: NextStatementAst emits a dedicated EXEC_NEXT opcode when no NEXTABLE ancestor exists, instead of throwing. The main input loop's next-record entry point (nextAddress) is registered as a property of the tuple stream, like the per-file addresses. Direct uses in rules keep today's behavior: a plain jump in input rules, compile-time rejection in special rules.
• Opcode: new EXEC_NEXT opcode, appended at the end of the enum so existing ordinals stay stable for precompiled tuple files.
• AwkTuples: nextAddress property (setter/getter, optimizer remap, dead-code reachability seed) and execNext(); serialVersionUID bumped to 7.
• AVM: EXEC_NEXT performs the gawk runtime rule checks — fatal error when called from a BEGIN, END, BEGINFILE, or ENDFILE rule, with gawk's exact messages (verified against gawk 5.0) — then unwinds the function calls via resetCallState() (which also settles the --profile timing stack, see --profile: function timing stack leaks on next/nextfile inside a function #557) and jumps to the main input loop's next-record address. A new mainInputLoopStarted flag (set by CONSUME_INPUT/CONSUME_FILE_INPUT) distinguishes BEGIN-time calls from input-rule calls.

$ printf 'a\nb\n' | jawk 'function f() { next } { f(); print "unreached" } END { print "end" }'
end
$ printf 'x\n' | jawk 'function f() { next } { } END { f() }'
AwkRuntimeException (line 1): `next' cannot be called from a `END' rule

Tests

New NextStatementTest covers: record skipping, nested call unwinding, later-rule bypass, per-file loop interplay with BEGINFILE/ENDFILE, runtime fatals from BEGIN/END/BEGINFILE/ENDFILE callers (including a program with no main input loop), and the unchanged compile-time rejection of direct uses in special rules. CliOptionTest gains a --profile test asserting functions unwound by next are recorded correctly (companion to the #557 nextfile test).

Documentation

• behavior-changes.md: bullet under Unreleased.
• cli.md: updated the next/nextfile/getline restrictions paragraph.

🤖 Generated with Claude Code

[bertysentry]

@codex please review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Hooray!

Reviewed commit: c81bf8208b

ℹ️ About Codex in GitHub

Your team has set up Codex to review pull requests in this repo. Reviews are triggered when you

• Open a pull request for review
• Mark a draft as ready
• Comment "@codex review".

If Codex has suggestions, it will comment; otherwise it will react with 👍.

Codex can also answer questions or update the PR. Try commenting "@codex address that feedback".