[silvervest]

Merge order: first of a stack of three. #179 and #180 build on this PR (they carry its commits until it merges), and #185 needs it too.

Bug fix.

What a fork's judge and brief told agents that wasn't so, found on an x86 game started from scratch:

• A unit built in steps counted as building when a step failed. failed_units matched the build's failed targets against each unit's object, but with ninja -k 0 a failed step means the object is never tried. An edit_file that broke a unit's stand-ins was kept as breaking nothing; the next compile found the unit broken, was told it had been broken before, and the agent lost its claim: 302 times over 156 functions in a week. failed_units now follows every target made from a failed one (ninja -t query) up to the unit's object.
• A compile that breaks a matched neighbour lost its own score. Where a function's code changes how its neighbours compile (inlined into them, or whole-program optimisation), a draft short of exact breaks a matched neighbour and the compile is undone. The rejection now gives the function's own score and diff, says the neighbours match again once it matches exactly, and records the attempt (never as the best attempt, which would be put back in the file). Agents of 63 functions had written about this.
• NOT IN YOUR BUILD named the row, not the symbol. A function whose row name was made unique with a trailing ' was reported as "its object doesn't define _f@4'"; agents took the quote for the defect and stopped looking.
• Code an #if leaves out read as the function's. Agents parked drafts of 13 functions (about 18 KB) in #if defined(SOME_FLAG) blocks no build defines, often because, where the build sees them, they break a matched neighbour. The brief showed each as the function's current source with its score, and every compile replaced the definition inside the block, where the judge never saw it. csource.hidden_by follows a file's conditionals and says which branch leaves a position out, only when it can be sure (ForkProject.never_defined: macros nothing could define; only looked for when code would otherwise be hidden). The brief marks a hidden definition, and a compile puts the agent's version where the build sees it, by its place in the original, without the parked copy, carrying the copy's reccmp annotation and comment along.
• (The rejection's list of broken functions no longer reuses the variable holding the function's names.)

Tests: test_a_unit_whose_build_step_fails_counts_as_not_building, test_not_in_your_build_names_the_symbol_not_the_rows_unique_name, test_which_if_branch_leaves_code_out_is_told_only_when_certain and test_a_macro_counts_as_never_defined_only_when_nothing_could_define_it (tests/test_forks.py); test_a_change_that_breaks_a_neighbour_still_says_how_close_the_function_came and test_a_definition_the_build_leaves_out_is_said_and_compiled_where_the_build_sees_it (tests/test_fork_judge.py). ruff check src tests projects is clean.

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