Fix immediate crash in Trim Trailing Spaces

[rainman74]

Motivation

• The Trim Trailing Spaces action was causing an immediate application crash due to brittle pointer arithmetic while computing and removing trailing spaces.
• A previous change still reused a single mutable pointer for multiple roles which left room for invalid pointer math and triggered the reported crash.

Description

• Rewrote trimtrailingspace to use explicit pointers lineend and trimstart instead of reusing the scan pointer, making the roles of source and destination positions distinct.
• Compute the trailing-space length as lold = (unsigned)(lineend - trimstart) and call memmovetest with trimstart + lnew as destination and lineend as source to make source/destination math explicit.
• Update the loop to advance with d = memspn(lineend, end, "\r\n", 2) and set lineend = trimstart after a move to preserve trimming semantics without unsafe arithmetic.

Testing

• No automated tests were executed in this environment.

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Codex Task