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Fix currentLine collapsing to 0 during for/range() loop synthetic steps (#234)

* Fix currentLine collapsing to 0 during for/range() loop synthetic steps

context.runtime.nodes[0]?.startToken?.line was read directly, with no
guard for runtime-fabricated nodes (implicit range() start/step bounds,
the per-iteration loop-variable increment). Those synthetic tokens are
always constructed with line: 0, since they don't correspond to any
line the user wrote. Real tokens are always 1-based (see
parser/token-bridge.ts), so line 0 is an unambiguous "not a real line"
signal — no dependency on any synthetic-token metadata is needed to
detect it.

generateForIncrement runs on every iteration of every for/range() loop,
so this wasn't a rare corner case: currentLine would collapse to 0 for
roughly half the steps of even a trivial 3-iteration loop, and (worse)
the for-line's reported line never recovered after the loop's first
iteration — every subsequent condition re-check and increment
overwrote it with 0 instead.

Fix: track the last real (line > 0) line seen and keep reporting it
whenever the current node's own line is 0, instead of overwriting with
the meaningless synthetic value.

Fixes #233.

* Give for/range() loop's synthetic bookkeeping nodes the real for-line

The previous fix (keeping the last real line while a synthetic node is
evaluating) eliminated the currentLine: 0 bug, but produced sticky
behavior: since the loop's condition-recheck and increment machinery
is entirely synthetic from the second iteration onward, currentLine
would latch onto the last real line seen (the loop body) and never
show the for-line again, even while the loop's own condition/increment
was conceptually what was executing.

These synthetic nodes (evaluateForIterator's implicit range() bounds,
generateForIncrement, and the FOR instruction handler's per-iteration
nodes) are logically part of evaluating the for-statement, so give them
its real line instead of 0 or nothing. curren... (continued)

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