[turbopack] correct a fencepost error in our inline string descision (#93524)
## What?
Fix an off-by-one in the `RcStr` inline-length check. Strings of length
exactly `MAX_INLINE_LEN` should be stored inline, not on the heap.
## Why?
`<` should have been `<=`. The buffer holds `MAX_INLINE_LEN` bytes and
the 4-bit tag field can encode that length, so the strict comparison was
wasting one byte of inline capacity for no reason — 7-byte strings were
allocating on the default 64-bit build. Bug dates back to #74482, the
original inline-string change.
This covers fun things like: "default" (×11), "project" (×8), "process"
(×8), "postcss" (×7), "require" (×6), "browser" (×4)
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