[rust-compiler] Fix off-by-one in SWC source-location conversion

[poteto]

SWC's BytePos is 1-based (BytePos(0) is the DUMMY/synthetic sentinel)
while the Babel AST that the rest of the compiler consumes uses
0-based offsets in loc.{start,end}.{line,column,index}. position()
in the SWC convert_ast was binary-searching the 1-based offset
against the 0-based line_offsets table, producing line/column/index
values that were off by 1, and that flipped to "next line, column 0"
when the referenced byte happened to be the byte right before a \n.

Shift the offset down by 1 before the line lookup. BaseNode.start/end
keeps the SWC-native 1-based value because the SWC scope collector
keys its node_to_scope map on span.lo.0 and the HIR builder looks it
up via base.start.

Also drop the e2e harness's oneBasedColumns = variant === 'swc'
workaround that was masking part of this divergence.

Test plan:

• bash compiler/scripts/test-e2e.sh --variant swc:
  Before: Total 1682/1795 (113 failures)
  After: Total 1742/1795 (53 failures, 60 fixed)
• bash compiler/scripts/test-e2e.sh --variant babel: 1788/1795 (unchanged)
• bash compiler/scripts/test-e2e.sh --variant oxc: 1702/1795 (unchanged)
• cargo test --workspace: 56 passed, 0 failed (unchanged)