feat: enable sbend="bend_s" in route_bundle strategies

[joamatab]

Summary

• Enable sbend="bend_s" in all route_bundle routing strategies
• This allows the router to use S-bends when needed to avoid collisions

Test plan

• Verify routing still works correctly with sbend enabled

Summary by Sourcery

Enhancements:

• Update the default route_bundle configuration to allow S-bend usage for improved collision avoidance in routing.

[sourcery-ai]

Reviewer's guide (collapsed on small PRs)

Reviewer's Guide

Configures route_bundle to pass sbend="bend_s" to gf.routing.route_bundle so that S-bends can be used in bundle routing while keeping other routing behavior unchanged.

Sequence diagram for updated route_bundle call with sbend bend_s

sequenceDiagram
    participant UserCode
    participant ubcpdk_tech as ubcpdk_tech
    participant gf_routing as gf_routing

    UserCode->>ubcpdk_tech: route_bundle(ports, separation, other_args)
    note right of ubcpdk_tech: route_bundle is a partial of gf.routing.route_bundle
    ubcpdk_tech->>gf_routing: route_bundle(ports, separation, cross_section=strip, sbend=bend_s, other_args)
    gf_routing-->>ubcpdk_tech: routes_with_optional_sbends
    ubcpdk_tech-->>UserCode: routes_with_optional_sbends

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Flow diagram for route_bundle configuration with sbend bend_s

flowchart LR
    A[ubcpdk_tech route_bundle definition] --> B[partial of gf.routing.route_bundle]
    B --> C[cross_section=strip]
    B --> D[sbend=bend_s]
    C --> E[User calls ubcpdk_tech.route_bundle]
    D --> E
    E --> F[gf.routing.route_bundle executes with S-bend capability]

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File-Level Changes

Change	Details	Files
Enable S-bend support for route_bundle by default.	Update the route_bundle partial to include sbend="bend_s" when calling gf.routing.route_bundle Keep cross_section configuration consistent with existing route_single and route_bundle_rib helpers	ubcpdk/tech.py

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[sourcery-ai]

Hey - I've left some high level feedback:

• Hardcoding sbend="bend_s" into the shared route_bundle partial reduces configurability; consider either exposing sbend as an argument to callers (with bend_s as the default) or defining a separate helper for S-bend routing to avoid surprising behavior changes for existing users of route_bundle.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- Hardcoding `sbend="bend_s"` into the shared `route_bundle` partial reduces configurability; consider either exposing `sbend` as an argument to callers (with `bend_s` as the default) or defining a separate helper for S-bend routing to avoid surprising behavior changes for existing users of `route_bundle`.

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[gemini-code-assist]

Code Review

This pull request modifies the route_bundle partial in ubcpdk/tech.py to include sbend="bend_s" by default. Feedback indicates that this global change may lead to cross-section mismatches or DRC errors in derived routing strategies, such as rib or metal routes, where curved S-bends are inappropriate.

[gemini-code-assist]

Applying sbend="bend_s" to the base route_bundle partial propagates this parameter to all derived routing strategies, including route_bundle_rib, route_bundle_metal, and route_bundle_metal_corner (defined in lines 345-364).

This introduces two potential issues:

1. Cross-section Mismatch: If bend_s is a fixed-layer component (common in PDKs using GDS cells like SiEPIC), it will cause layer or width mismatches when used in route_bundle_rib or route_bundle_metal.
2. Electrical Routing: Electrical strategies like route_bundle_metal typically avoid curved S-bends. Enabling this globally may cause the router to place optical silicon components in metal routes, leading to DRC errors.

Consider enabling sbend only for the specific optical strip strategy or ensuring that the electrical and rib strategies explicitly override it with sbend=None.