Attribute sweep, compileProject, and a canary for check_examples

[KubaO]

Adds a tool that asks the compiler where every attribute is legal, moves tbbuild into a library the harness tools call directly, and stops check_examples from believing an IDE that has not yet reported.

• scripts: compileProject (scripts/lib/tb-build.mjs) is tbbuild as a function; tbbuild, check_examples and the sweep call it instead of spawning a process. tbbuild's output is byte-identical on seven cases. The IDE shuts down asynchronously, so one lane's shutdown no longer freezes the others.
• scripts: sweep_attributes (new) builds every attribute at every one of 62 declaration sites, beside a baseline with no attribute and a control with an invented name, and reports where the compiler accepts each one. A batch that crashes, hangs or disturbs its canaries is halved. On BETA 987: 34,526 probes, 8 to 16 minutes on four lanes, and 46 of 71 documented attributes disagree with the page. Many of those are "accepted but not documented" rather than wrong. Outside every gate, like examples.bat.
• test.bat: check_attribute_sweep (new, in the CI action and Tools.md) tests the sweep's logic with no IDE: 138 probes, and each of 23 injected faults fails one. test.bat is now sixteen steps.
• docs: ComExport is also accepted on API Declares; the Hidden note cites a real example. BUGS-TO-REPORT.md gains a two-line [PopulateFrom] compiler crash.
• check_examples: an IDE under load can read as idle with an empty Problems panel, and that read passed every sample in its batch. Every batch now carries a #Warning canary (TB0005, kept a warning by [EnforceWarnings]). A read with no errors must report it; a silent read is built again, then halved, and a unit that stays silent stops the run. A read with real errors is taken as read, so a failing corpus is never stopped or slowed.
• docs: Set.md's Form1 sample compiled only when batched beside another page's Class Form1; it has its own hidden class now.