Multiplayer: Add client-side build/sell previews and fix various issues

[rainlizard]

I've spotted a couple of issues with roomspace prediction. The biggest issue was that holding CTRL while holding a creature would display the 5x5 dig box even when the cursor was not over a slab. There were also a few smaller timing issues where the cursor box could switch between client-side prediction and authoritative rendering at the wrong time. Threw in some integer enum renames as well. I've refactored it to use more shared code instead of duplicated code, which will be more resilient for any future changes to roomspace that people might make. The tag/untag state also needed to be simplified.

The roomspace preview box is now 100% client-side for building/selling as well, previously it was only client-side for digging.

[Copilot]

Pull request overview

This PR refactors multiplayer roomspace/cursor preview handling to make build/sell previews fully client-side (similar to existing dig prediction) and to fix several timing/state issues in highlight/tag preview behavior.

Changes:

• Adds client-side build/sell roomspace preview updates and reuses shared roomspace/packet-action logic.
• Refactors dig tag/untag mode selection to be derived from roomspace state/predicted tag modes (removing the old flag-based set_tag_untag_mode path).
• Renames/clarifies integer “mode” usage by switching call sites to roomspace_*_mode enum values and consolidating packet action handling via apply_roomspace_packet_action().

Reviewed changes

Copilot reviewed 10 out of 10 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/roomspace.h	Updates APIs to pass PlayerInfo/Packet, adds preview update helpers, and exposes shared packet-action application.
src/roomspace.c	Implements shared dig tag-mode derivation, highlight refactor, build/sell preview helpers, and apply_roomspace_packet_action().
src/roomspace_prediction.c	Extends prediction pipeline to cover build/sell previews and refactors dig prediction to use the new shared APIs.
src/player_data.h	Removes unused PlaF_ChosenSlabHasActiveTask flag and simplifies swap_to_untag_mode field comment.
src/packets.h	Removes set_tag_untag_mode() declaration.
src/packets.c	Routes roomspace packet actions through apply_roomspace_packet_action() and updates mode comparisons to enums.
src/packets_input.c	Uses new preview helpers; removes set_tag_untag_mode() calls; updates dig cursor-tagging call signature.
src/engine_redraw.c	Updates pointer graphic selection to use drag_placement_mode enum value.
src/cursor_tag.h	Changes tag_cursor_blocks_dig() signature to accept PlayerInfo/Packet/RoomSpace.
src/cursor_tag.c	Updates implementation to use caller-provided packet/roomspace context (supporting prediction-driven rendering).

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