Fix exclusive execution of conditional branches

[pratheesh]

Fixes #17.

The register allocator now emits an explicit merge jump between the fall-through and target branches, so an If/Else graph executes exactly one branch without requiring manual Flow Control terminators.

Also adds a small headless simulator regression and CI workflow covering both taken and fall-through paths, and updates the block guidance.

Verification:

• node scripts/test_headless_sim.js
• AM243x conditional SysConfig generation + clpru compile/link
• git diff --check

[pratheesh]

Closing in favour of #19, which fixes #17 by making Flow Control terminators on connected branches a validation requirement instead of inserting a merge jump.

For reference, a gap found in this approach while comparing the two: when the If/Else sits inside a Loop block, the merge label was emitted after the loop's back-edge (QBA startloop_N), so the fall-through branch exited the loop. In tamagawa_single_channel the firmware halted after the first transaction. The explicit Flow Control → startloop_0 termination in #19's updated example avoids that.