fix(xmodem): truncate file on open instead of appending

[benallfree]

This PR fixes what I perceive is a bug in the XModem module: subsequent file writes to the same path append data rather than truncating.

🤝 Attestations

• I have tested that my proposed changes behave as described.
• I have tested that my proposed changes do not cause any obvious regressions on the following devices:
  • Heltec (Lora32) V3
  • LilyGo T-Deck
  • LilyGo T-Beam
  • RAK WisBlock 4631
  • Seeed Studio T-1000E tracker card
  • Other (please specify below)

[Copilot]

Pull request overview

This PR updates the XModem receive path to ensure a new transfer to an existing filename overwrites prior contents (truncate semantics) rather than appending, aligning expected file-transfer behavior across supported filesystems/platforms.

Changes:

• Delete any pre-existing destination file before opening for write in the XModem receive flow.

[benallfree]

@thebentern Copilot brought up a good point about preserving atomicity by writing to a tmp file and then doing a rename. I updated the PR to reflect that.

[Copilot]

Pull request overview

This PR updates the XModem receive flow to avoid unintended appending when receiving a file to an existing path by writing to a temporary file and committing it at end-of-transmission.

Changes:

• Add receive state (recvCommitPending) and a receive temp path buffer (recvTmpPath) to support temp-file based receives.
• On receive start, build {filename}.tmp, remove any pre-existing temp file, and write incoming blocks to the temp file.
• On EOT/CAN, close and either commit via renameFile() (with retry support) or delete the temp file.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/xmodem.h	Adds new receive state and temp path storage to support safe receive/commit behavior.
src/xmodem.cpp	Implements temp-file receive + EOT commit (rename) and cancel cleanup; improves filename copying/termination.

[benallfree]

@thebentern Sorry for the delay, for some reason I am not getting GH notifications on this PR.